As filed with the Securities and Exchange Commission on December 28, 2022

1933 Act File No. 33-7638

1940 Act File No. 811-4777

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER THE SECURITIES ACT OF 1933

POST-EFFECTIVE AMENDMENT NO. 89

AND

REGISTRATION STATEMENT

UNDER THE INVESTMENT COMPANY ACT OF 1940

AMENDMENT NO. 91

MFS^® SERIES TRUST I

(Exact Name of Registrant as Specified in Charter)

111 Huntington Avenue, Boston, Massachusetts 02199-7632

(Address of Principal Executive Offices)

Registrant’s Telephone Number, including Area Code: 617-954-5000

Christopher R. Bohane, Massachusetts Financial Services Company,

111 Huntington Avenue, Boston, MA 02199-7632

(Name and Address of Agent for Service)

Approximate Date of Proposed Public Offering:

It is proposed that this filing will become effective (check appropriate box)

☐ immediately upon filing pursuant to paragraph (b)

☒ on December 29, 2022, pursuant to paragraph (b)

☐ 60 days after filing pursuant to paragraph (a)

☐ on [date], pursuant to paragraph (a)

☐ 75 days after filing pursuant to paragraph (a)(2)

☐ on [date], pursuant to paragraph (a)(2) of rule 485

If appropriate, check the following box:

☐ this post-effective amendment designates a new effective date for a previously filed post-effective amendment

December 29, 2022

PROSPECTUS

MFS^® Core Equity Fund

The investment objective of the fund is to seek capital appreciation.

Table of contents

The Securities and Exchange Commission has not approved or disapproved the fund’s shares or determined whether this prospectus is accurate or complete. Anyone who tells you otherwise is committing a crime.

RGI-PRO-122922

MFS Core Equity Fund

Summary of Key Information

Investment Objective

Fees and Expenses

Shareholder Fees (fees paid directly from your investment):

MFS Core Equity Fund

Example

Although your actual costs will likely be higher or lower, under these assumptions your costs would be:

Portfolio Turnover

Principal Investment Strategies

Principal Risks

MFS Core Equity Fund 

Performance Information

Performance Table.

Average Annual Total Returns

(For the Periods Ended December 31, 2021)

Investment Adviser

MFS serves as the investment adviser for the fund.

Portfolio Manager(s)

Purchase and Sale of Fund Shares

You may purchase and redeem shares of the fund each day the New York Stock Exchange (the NYSE) is open for trading. You may purchase or redeem shares either by having your financial intermediary process your purchase or redemption, or through MFS Service Center, Inc. (MFSC) by overnight mail (MFSC, Suite 219341, 430 W 7^th Street, Kansas City, MO 64105-1407), by mail ([Fund Name], P.O. Box 219341, Kansas City, MO 64121-9341), by telephone (1-800-225-2606), or via the Internet at mfs.com (MFS Access).

The fund’s initial and subsequent investment minimums generally are as follows:

MFS Core Equity Fund

Purchases of Class B shares are closed to new and existing investors except through reinvestment of dividends and capital gain distributions. Existing investors may continue to exchange their Class B shares for the same share class of another MFS fund.

Taxes

If your shares are held in a taxable account, the fund’s distributions will be taxed to you as ordinary income and/or capital gains. If your shares are held in a tax-advantaged account, you will generally be taxed only upon withdrawals from the account.

Payments to Broker/Dealers and Other Financial Intermediaries

If you purchase shares of the fund through a broker/dealer or other financial intermediary (such as a bank), the fund, MFS, and/or MFS’ affiliates may pay the financial intermediary for the sale of shares of the fund and/or the servicing of shareholder accounts. These payments may create a conflict of interest by influencing your broker/dealer or other financial intermediary and your salesperson to recommend the fund over another investment. Ask your financial intermediary or visit your financial intermediary’s website for more information.

Investment Objective, Strategies, and Risks

Investment Objective

The fund’s investment objective is to seek capital appreciation. The fund’s objective may be changed without shareholder approval.

Principal Investment Strategies

MFS normally invests at least 80% of the fund’s net assets in equity securities.

In selecting investments for the fund, MFS is not constrained by any particular investment style. MFS may invest the fund’s assets in the stocks of companies it believes to have above average earnings growth potential compared to other companies (growth companies), in the stocks of companies it believes are undervalued compared to their perceived worth (value companies), or in a combination of growth and value companies.

MFS may invest the fund’s assets in securities of companies of any size.

MFS may invest the fund’s assets in foreign securities.

MFS normally invests the fund’s assets across different industries and sectors, but MFS may invest a significant percentage of the fund’s assets in issuers in a single industry or sector.

A team of investment research analysts selects investments for the fund. MFS generally expects the fund’s exposure to broad industry categories to approximate the exposure of the Russell 3000^® Index to these broad industry categories using MFS’ custom industry categories to classify the fund and the Russell 3000^® Index’s holdings.

MFS uses an active bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual issuers and their potential in light of their financial condition, and market, economic, political, and regulatory conditions. Factors considered may include analysis of an issuer’s earnings, cash flows, competitive position, and management ability. MFS may also consider environmental, social, and governance (ESG) factors in its fundamental investment analysis where MFS believes such factors could materially impact the economic value of an issuer. ESG factors considered may include, but are not limited to, climate change, resource depletion, an issuer’s governance structure and practices, data protection and privacy issues, and diversity and labor practices. Quantitative screening tools that systematically evaluate an issuer’s valuation, price and earnings momentum, earnings quality, and other factors, may also be considered.

For purposes of the fund’s 80% policy, net assets include the amount of any borrowings for investment purposes.

Principal Investment Type

The principal investment type in which the fund may invest is:

Equity Securities: Equity securities represent an ownership interest, or the right to acquire an ownership interest, in a company or other issuer. Different types of equity securities provide different voting and dividend rights and priorities in the event of bankruptcy of the issuer. Equity securities include common stocks, preferred stocks, securities convertible into stocks, equity interests in real estate investment trusts, and depositary receipts for such securities.

Principal Risks

The share price of the fund will change daily based on changes in market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. The significance of any specific risk to an investment in the fund will vary over time depending on the composition of the fund’s portfolio, market conditions, and other factors. You should read all of the risk information below carefully, because any one or more of these risks may result in losses to the fund.

The principal risks of investing in the fund are:

Investment Selection Risk: MFS’ investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests. In addition, MFS or the fund’s other service providers may experience disruptions or operating errors that could negatively impact the fund.

Equity Market Risk: Equity markets can be volatile and can decline significantly in response to changes in, or investor perceptions of, issuer, market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. These conditions can affect a single issuer or type of security, issuers within a broad market sector, industry or geographic region, or the equity markets in general. Different parts of the market and different types of securities can react differently to these conditions. For example, the stocks of growth companies

MFS Core Equity Fund

can react differently from the stocks of value companies, and the stocks of large cap companies can react differently from the stocks of small cap companies. Certain changes or events, such as political, social, or economic developments, including increasing or negative interest rates or the U.S. government’s inability at times to agree on a long-term budget and deficit reduction plan (which has in the past resulted and may in the future result in a government shutdown); market closures and/or trading halts; government or regulatory actions, including the imposition of tariffs or other protectionist actions and changes in fiscal, monetary, or tax policies; natural disasters; outbreaks of pandemic and epidemic diseases; terrorist attacks; war; and other geopolitical changes or events, can have a dramatic adverse effect on equity markets and may lead to periods of high volatility in an equity market or a segment of an equity market.

Company Risk: Changes in the financial condition of a company or other issuer, changes in specific market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions that affect a particular type of investment or issuer, and changes in general market, economic, political, regulatory, geopolitical, environmental, public health, and other conditions can adversely affect the prices of investments. The value of an investment held by the fund may decline due to factors directly related to the issuer, such as competitive pressures, cybersecurity incidents, financial leverage, historical and/or prospective earnings, management performance, labor and supply shortages, investor perceptions, and other factors. The prices of securities of smaller, less well-known issuers can be more volatile than the prices of securities of larger issuers or the market in general.

Foreign Risk: Investments in securities of foreign issuers, securities of companies with significant foreign exposure, and foreign currencies can involve additional risks relating to market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. Political, social, diplomatic, and economic developments, U.S. and foreign government action, or the threat thereof, such as the imposition of currency or capital blockages, controls, or tariffs, economic and trade sanctions or embargoes, security trading suspensions, entering or exiting trade or other intergovernmental agreements, or the expropriation or nationalization of assets in a particular country, can cause dramatic declines in certain or all securities with exposure to that country and other countries. In the event of nationalization, expropriation, confiscation or other government action, intervention, or restriction, the fund could lose its entire investment in a particular foreign issuer or country. Economies and financial markets are interconnected, which increases the likelihood that conditions in one country or region can adversely impact issuers in different countries and regions. Less stringent regulatory, accounting, auditing, and disclosure requirements for issuers and markets are more common in certain foreign countries. Enforcing legal rights can be difficult, costly, and slow in certain foreign countries and with respect to certain types of investments, and can be particularly difficult against foreign governments. Changes in currency exchange rates can significantly impact the financial condition of a company or other issuer with exposure to multiple countries as well as affect the U.S. dollar value of foreign currency investments and investments denominated in foreign currencies. Additional risks of foreign investments include trading, settlement, custodial, and other operational risks, and withholding and other taxes. These factors can make foreign investments, especially those tied economically to emerging markets, more volatile and less liquid than U.S. investments. In addition, foreign markets can react differently to market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions than the U.S. market.

Focus Risk: Issuers in a single industry, sector, country, or region can react similarly to market, currency, political, economic, regulatory, geopolitical, environmental, public health, and other conditions. These conditions include business environment changes; economic factors such as fiscal, monetary, and tax policies; inflation and unemployment rates; and government and regulatory changes. The fund’s performance will be affected by the conditions in the industries, sectors, countries, and regions to which the fund is exposed.

Liquidity Risk: Certain investments and types of investments are subject to restrictions on resale, may trade in the over-the-counter market, or may not have an active trading market due to adverse market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions, including investors trying to sell large quantities of a particular investment or type of investment, or lack of market makers or other buyers for a particular investment or type of investment. At times, all or a significant portion of a market may not have an active trading market. Without an active trading market, it may be difficult to value, and it may not be possible to sell, these investments and the fund could miss other investment opportunities and hold investments it would prefer to sell, resulting in losses for the fund. In addition, the fund may have to sell certain of these investments at prices or times that are not advantageous in order to meet redemptions or other cash needs, which could result in dilution of remaining investors’ interests in the fund. The prices of illiquid securities may be more volatile than more liquid investments.

Large Shareholder Risk: From time to time, shareholders of the fund (which may include institutional investors, financial intermediaries, or other MFS funds) may make relatively large redemptions or purchases of fund shares. These transactions may cause the fund to sell securities or invest additional cash, as the case may be, at disadvantageous prices. While it is impossible to predict the overall impact of these transactions over time, there could be adverse effects on the fund’s performance to the extent that the fund may be required to sell securities or invest cash at times it would not otherwise do so. Redemptions of a large number of shares also may increase transaction and other costs or have adverse tax consequences for shareholders of the fund by requiring a sale of portfolio securities. In addition, a large redemption could result in the fund’s current expenses being allocated over a smaller asset base, leading to an increase in the fund’s expense ratio. Purchases of a large number of shares may adversely affect the fund’s performance to the extent that it takes time to invest new cash and the fund maintains a larger cash position than it ordinarily would.

Other Investment Strategies and Risks

Active and Frequent Trading: MFS may engage in active and frequent trading in pursuing the fund’s principal investment strategies. Frequent trading increases transaction costs, which can reduce the fund’s return. Frequent trading can also increase the possibility of capital gain and ordinary distributions. Frequent trading can also result in the realization of a higher percentage of short-term capital gains and a lower percentage of long-term capital gains as compared to a fund that trades less frequently. Because short-term capital gains are distributed as ordinary income, this would generally increase your tax liability unless you hold your shares through a tax-advantaged or tax-exempt vehicle.

Operational and Cyber Security Risk: The fund and its service providers, and your ability to transact with the fund, may be negatively impacted due to operational matters arising from, among other problems, human errors, systems and technology disruptions or failures, or cyber security incidents. Operational issues and cyber security incidents may cause the fund or its service providers, as

MFS Core Equity Fund

well as securities trading venues and other market participants, to suffer data corruption and/or lose operational functionality, and could, among other things, interfere with the processing of shareholder transactions, impair the ability to calculate the fund’s net asset value per share, impede trading of portfolio securities, and result in the theft, misuse, and/or improper release of confidential information relating to the fund or its shareholders. Such operational issues and cyber security incidents may result in losses to the fund and its shareholders.

Temporary Defensive Strategy: In response to adverse market, economic, industry, political, or other conditions, MFS may depart from the fund’s principal investment strategies by temporarily investing for defensive purposes. When MFS invests defensively, different factors could affect the fund’s performance and the fund may not achieve its investment objective. In addition, the defensive strategy may not work as intended.

Further Information on Investment Strategies, Types, and Risks: Information about investment strategies and investment types not described in the prospectus and the risks associated with those investment strategies and investment types are described in the fund’s Statement of Additional Information (SAI).

Management of the Fund

Investment Adviser

MFS, located at 111 Huntington Avenue, Boston, Massachusetts 02199-7618, serves as the investment adviser for the fund. Subject to the supervision of the fund’s Board of Trustees, MFS is responsible for managing the fund’s investments, executing transactions, and providing related administrative services and facilities under an Investment Advisory Agreement between the fund and MFS.

For the fiscal year ended August 31, 2022, the fund paid MFS an effective management fee equal to 0.51% of the fund’s average daily net assets.

The management fee set forth in the Investment Advisory Agreement is 0.65% of the fund’s average daily net assets annually up to $500 million; 0.55% of the fund’s average daily net assets annually in excess of $500 million and up to $2.5 billion; 0.50% of the fund’s average daily net assets annually in excess of $2.5 billion and up to $5 billion; 0.47% of the fund’s average daily net assets annually in excess of $5 billion and up to $10 billion; and 0.45% of the fund’s average daily net assets annually in excess of $10 billion.

MFS has agreed in writing to reduce its management fee by a specified amount if certain MFS mutual fund assets exceed thresholds agreed to by MFS and the fund’s Board of Trustees. Effective March 1, 2022, MFS has agreed in writing to waive at least 0.01% of its management fee as part of this agreement. The agreement to waive at least 0.01% of the management fee will continue until modified by the fund’s Board of Trustees, but such agreement will continue until at least December 31, 2023. For the fiscal year ended August 31, 2022, this management fee reduction amounted to 0.01% of the fund’s average daily net assets.

A discussion regarding the basis for the Board of Trustees’ approval of the Investment Advisory Agreement is available in the fund’s annual report for the one-year period ended August 31, 2022.

MFS is America’s oldest mutual fund organization. MFS and its predecessor organizations have a history of money management dating from 1924 and the founding of the first mutual fund, Massachusetts Investors Trust. Net assets under the management of the MFS organization were approximately $528 billion as of October 31, 2022.

Disclosure of Portfolio Holdings. The fund has established a policy with respect to the disclosure of fund portfolio holdings. A description of this policy is provided in the SAI.

The following information is available to you at mfs.com/openendfunds by selecting the fund’s name:

Holdings also include short positions, if any. Top 10 holdings exclude cash, cash equivalents, short-term investments, currency derivatives, and the cash portion of other derivatives. For purposes of full holdings, cash, cash equivalents, and short-term investments are aggregated and currency derivatives and the cash portion of other derivatives are aggregated.

Note that the fund or MFS may suspend the posting of this information or modify the elements of this Web posting policy without notice to shareholders. Once posted, the above information will generally remain available on mfs.com until at least the date on which the fund files a Form N-CSR or Form N-PORT for the period that includes the date as of which the mfs.com information is current.

Portfolio Manager(s)

Information regarding the portfolio manager(s) of the fund is set forth below. Further information regarding the portfolio manager(s), including other accounts managed, compensation, ownership of fund shares, and possible conflicts of interest, is available in the fund’s SAI.

Administrator

MFS provides the fund with certain financial, legal, and other administrative services under a Master Administrative Services Agreement between the fund and MFS. Under the Agreement, MFS is paid an annual fee for providing these services.

Distributor

MFS Fund Distributors, Inc. (MFD), a wholly-owned subsidiary of MFS, is the distributor of shares of the fund.

Shareholder Servicing Agent

MFS Service Center, Inc. (MFSC), a wholly-owned subsidiary of MFS, provides dividend and distribution disbursing and transfer agent and recordkeeping functions in connection with the issuance, transfer, and redemption of each class of shares of the fund under a Shareholder Servicing Agent Agreement. MFSC receives a fee based on the costs it incurs in providing these services and a target profit margin. In addition, MFSC is reimbursed for payments made to service providers that provide certain sub-accounting and other shareholder services (shareholder servicing payments) and its out-of-pocket expenses. No shareholder servicing payments are made for Class R6 shares.

MFS Core Equity Fund

Description of Share Classes

The fund offers Class A, Class B, Class C, Class I, Class R1, Class R2, Class R3, Class R4, and Class R6 shares through this prospectus. All classes of the fund have the same investment objective and investments, but each class has its own sales charge and expense structure. Your financial intermediary may also charge you additional fees, commissions, or other charges. You should consult with your financial intermediary to help you determine which class is most appropriate for you.

Purchases of Class B shares are closed to new and existing investors except through reinvestment of dividends and capital gain distributions. Existing investors may continue to exchange their Class B shares for the same share class of another MFS fund.

Share Class Eligibility

Your financial intermediary is responsible for determining if you meet any share class eligibility requirements. MFS does not generally monitor any share class eligibility requirements.

Class A, Class B, and Class C shares do not have any share class eligibility requirements.

Class I shares generally are available only to the following eligible investors:

Class I shares are also available to other institutional investors who have entered into an agreement with MFD to purchase Class I shares and investors who purchase shares through brokerage programs of certain financial intermediaries that have entered into an agreement with MFD to offer Class I shares through such programs. In addition, Class I shares are available to investors whose Class A, Class B, or Class C shares have been converted to Class I shares as a result of no financial intermediary being specified on the investor’s account. (Please see “Conversion Among Share Classes” below.)

Class R1, Class R2, Class R3, and Class R4 shares generally are available only to eligible retirement plans (401(k) plans, 457 plans, employer-sponsored 403(b) plans, profit sharing and money purchase pension plans, defined benefit plans and non-qualified deferred compensation plans any of whose accounts are maintained by the fund at an omnibus level (“Employer Retirement Plans”)), and health savings accounts under Section 223 of the Internal Revenue Code if such accounts are maintained by the fund at an omnibus level.

Class R1, Class R2, Class R3, and Class R4 shares generally are not available to traditional and Roth IRAs, Coverdell Educational Savings Accounts, SEPs, SAR-SEPs, SIMPLE IRAs, salary reduction only 403(b) plans, and Section 529 tuition plans.

Class R6 shares generally are available only to the following eligible investors:

Class R6 shares are also available to other institutional investors who have entered into an agreement with MFD to purchase Class R6 shares and investors who purchase shares through brokerage programs of certain financial intermediaries that have entered into an agreement with MFD to offer Class R6 shares through such programs.

Conversion Among Share Classes

If you hold Class B shares for approximately eight years, they will convert to Class A shares of the fund. For purposes of calculating the conversion date, purchases made on any day during a calendar month will age one month at the close of business on the last day of that month, and at the close of business on the last day of each subsequent month.

All Class B shares you acquire through the reinvestment of dividends and distributions will be held in a separate sub-account. Each time any Class B shares in your account convert to Class A shares, a proportionate number of the Class B shares in the sub-account will also convert to Class A shares.

For shareholders invested in Class B shares through a financial intermediary, it is the responsibility of the financial intermediary to ensure that the investor is credited with the proper holding period. The automatic conversion of Class B shares to Class A shares will not apply to shares held through financial intermediaries that do not track the length of time that a shareholder has held such shares. Please consult your financial intermediary for more information.

If you hold Class C shares for approximately eight years, they will convert to Class A shares of the fund. For purposes of calculating the conversion date, purchases made on any day during a calendar month will age one month at the close of business on the last day of that month, and at the close of business on the last day of each subsequent month.

All Class C shares you acquire through the reinvestment of dividends and distributions will be held in a separate sub-account. Each time any Class C shares in your account convert to Class A

MFS Core Equity Fund

shares, a proportionate number of the Class C shares in the sub-account will also convert to Class A shares.

For shareholders invested in Class C shares through a financial intermediary, it is the responsibility of the financial intermediary to ensure that the investor is credited with the proper holding period. The automatic conversion of Class C shares to Class A shares will not apply to shares held through financial intermediaries that do not track the length of time that a shareholder has held such shares. In addition, for shareholders invested in Class C shares through a financial intermediary, Class C shares may be automatically exchanged for Class A shares of the fund under the policies of the financial intermediary, as described in Appendix A of this prospectus.  It is solely the responsibility of the respective financial intermediary to administer and support such transactions. Please consult your financial intermediary for more information.

Class C shareholders of the fund may be able to convert their shares to Class A shares of the fund if their financial intermediary provides written notification to MFD prior to such conversion that the intermediary has determined that Class A was the appropriate share class at the time of purchase for such shareholder and receives approval of such conversion by MFD. Class C shareholders also may be able to convert their Class C shares not subject to a CDSC to Class I shares of the fund if they satisfy eligibility requirements of Class I shares.

Shareholders may be able to convert between Class A shares and Class I shares of the fund if they satisfy eligibility requirements for the other class, if any. Class I shareholders may be able to convert their Class I shares to Class R6 shares of the fund if they satisfy eligibility requirements of Class R6 shares.

Class A, Class B, or Class C shares in an account held directly with MFSC for which no financial intermediary is specified will be periodically converted to Class I shares as authorized by MFD. If your Class A, Class B, or Class C shares are converted to Class I shares pursuant to this conversion feature, all future purchases in your account will be invested in Class I shares. Shareholders will generally be provided with written notice at least 30 days prior to a conversion of their Class A, Class B, or Class C shares to Class I shares.

If a shareholder converts from one share class to another share class of the fund, as described above, the transaction will be based on the respective net asset value of each class as of the trade date for the conversion. Consequently, the converting shareholder may receive fewer shares or more shares than originally owned, depending on that day’s net asset values. The total value of the initially held shares, however, will equal the total value of the converted shares received. A conversion between share classes in the same fund, as described above, is a nontaxable event for federal income tax purposes.

Sales Charges and Waivers and Reductions

You may be subject to an initial sales charge when you purchase Class A shares, or a CDSC when you redeem Class A, Class B, or Class C shares. These sales charges are paid to MFD.

You may qualify for a sales charge waiver or reduction when you purchase or redeem shares. The waivers or reductions that are applicable to your transaction depend on your financial intermediary and the type of account and transaction. Further details regarding these waivers and reductions are provided in Appendix A of this prospectus. It is your responsibility to inform your financial intermediary or MFSC upon purchasing fund shares of your eligibility for a sales charge waiver or reduction. Waivers or reductions may be eliminated, modified, and added at any time without providing advance notice to shareholders.

Class A Shares. You may purchase Class A shares at the offering price (which includes the applicable initial sales charge).

The amount of the initial sales charge you pay when you buy Class A shares differs depending upon the amount you invest, as follows:

You may purchase Class A shares without an initial sales charge when you invest $1 million or more in Class A shares. However, for purchasers other than Employer Retirement Plans, a CDSC of 1% will generally be deducted from your redemption proceeds if you redeem within 18 months of purchase.

Purchases of the fund and exchanges out of the fund are subject to the MFS funds’ policies concerning disruptive trading practices. This includes exchanges out of the fund within a short period of time after purchase for the purpose of avoiding a higher initial sales charge on the MFS fund into which you exchange. The MFS funds reserve the right to restrict, reject, or cancel, without any prior notice, any purchase or exchange order believed to represent frequent or other disruptive trading. In such situations, the MFS funds may prohibit future purchases from you and/or the financial intermediary associated with your account. See “How to Purchase, Redeem, and Exchange Shares—Disruptive Trading” for more information regarding MFS funds’ policies concerning disruptive trading practices.

Class B Shares. You may purchase Class B shares at net asset value without an initial sales charge, but if you redeem your shares within the first six years after purchase, you may be subject to a CDSC (declining from 4% during the first year to 0% after six years).

The CDSC is imposed according to the following schedule:

Class C Shares. You may purchase Class C shares at net asset value without an initial sales charge. However, a CDSC of 1% will generally be deducted from your redemption proceeds if you redeem within 12 months of your purchase.

Class I Shares. Eligible investors may purchase Class I shares at net asset value without an initial sales charge or a CDSC upon redemption.

Class R1, Class R2, Class R3, Class R4, and Class R6 Shares. Eligible investors may purchase Class R1, Class R2, Class R3,

MFS Core Equity Fund

Class R4, and Class R6 shares at net asset value without an initial sales charge or a CDSC upon redemption.

Calculation Of CDSC. As discussed above, certain investments in Class A, Class B, and Class C shares are subject to a CDSC. For purposes of calculating the CDSC, purchases made on any day during a calendar month will age one month at the close of business on the last day of that month, and at the close of business on the last day of each subsequent month. The CDSC is based on the original purchase cost or the current market value of the shares being sold, whichever is less. For purposes of determining the CDSC, if you sell only some of your shares, shares not subject to a CDSC are sold first, followed by shares held the longest.

Shares acquired through reinvestment of distributions are not subject to a CDSC.

Distribution and Service Fees

The fund has adopted a plan in accordance with Rule 12b-1 under the Investment Company Act of 1940, as amended (the “Distribution Plan”). Under the Distribution Plan, the fund pays distribution and/or service fees to MFD to support the sale and distribution of Class A, Class B, Class C, Class R1, Class R2, and Class R3 shares, and/or shareholder servicing and account maintenance activities. These distribution and/or service fees equal on an annual basis up to the following maximum percentages of average daily net assets of the class:

These fees are paid out of fund assets of the applicable class of shares. Because these fees are an ongoing expense of the fund, they increase the cost of your investment over time and may cost you more than other types of sales charges.

MFD has voluntarily agreed to rebate to the class a portion of such class’ 0.25% service fee attributable to accounts for which there is no financial intermediary specified on the account except for accounts attributable to seed money of MFS or an affiliate.

The fund has not adopted a Rule 12b-1 plan with respect to its Class I, Class R4, or Class R6 shares.

Financial Intermediary Compensation

The term “financial intermediary” refers to any broker/dealer, bank (including bank trust departments), registered investment adviser, financial planner, retirement plan administrator, third-party administrator, insurance company, consultant, and any other institution having a selling, administration, or any similar agreement with MFD, MFS, or any of their affiliates.

Financial intermediaries receive various forms of compensation in connection with the sale of shares of a fund and/or the servicing of shareholder accounts. Financial intermediaries receive such compensation (i) in the form of up-front commissions and ongoing asset-based compensation paid by MFD based on sales charges received and expected to be received by MFD from shareholders and Distribution Plan distribution and service payments received by MFD from the fund, (ii) in the form of shareholder servicing payments paid by an affiliate of MFD based on the receipt of such payments by MFD or its affiliates from the fund, and/or (iii) in the form of payments paid by MFD from MFD’s own additional resources.

Financial intermediaries receive up-front commissions of up to the following percentage amount(s) for sales of the following share class(es):

In addition, certain financial intermediaries receive payments from MFD from MFD’s own additional resources as incentives to market the MFS funds, to participate in MFD’s promotional efforts and/or in recognition of their marketing support, administrative services, and/or transaction processing support. This compensation from MFD is not reflected in the “Fees and Expenses” table in the fund’s prospectus. MFD compensates financial intermediaries based on criteria established by MFD from time to time, including the distribution potential of the financial intermediary, the types of products and programs offered by the financial intermediary, eligibility for placement on the financial intermediary’s preferred or recommended list, the financial and contractual terms with the financial intermediary, the level and/or type of marketing and administrative support provided by the financial intermediary, and the quality of the overall relationship with the financial intermediary. In particular, MFD normally considers the level of assets attributable to the financial intermediary, the level of redemptions by the financial intermediary, the level of access to the financial intermediary’s representatives and management, the ability to educate the representatives of the financial intermediary about the MFS funds, and the number of representatives of the financial intermediary potentially utilizing the MFS funds with their clients.

MFD makes these additional payments to financial intermediaries for marketing support and administrative services with respect to fund shares sold or held through the financial intermediary’s distribution network and/or through programs such as retirement programs, qualified tuition programs, fund supermarkets, fee-based advisory or wrap fee programs, bank trust programs, and insurance (e.g., individual or group annuity) programs. MFD also makes payments to certain financial intermediaries to help offset the costs associated with client account maintenance support, statement preparation, and transaction processing. To the extent permitted by U.S. Securities and Exchange Commission (SEC) and Financial Industry Regulatory Authority (FINRA) rules and other applicable laws and regulations, MFD makes other payments or allows other promotional incentives or payments to financial intermediaries.

The types of payments described above are not exclusive and such payments can be significant to the financial intermediary. In addition, the compensation that financial intermediaries receive may vary by class of shares sold and among financial intermediaries. Depending upon the arrangements in place at any particular time, financial intermediaries may have a financial incentive to recommend a particular fund or share class when a less expensive option with the same or similar characteristics is available or to recommend MFS funds instead of other funds that generate less or no compensation for the financial intermediary. Likewise, such compensation could create incentives for the financial intermediary to hold an investment longer than might be appropriate, or

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conversely, to make more frequent exchanges or other transactions than might be necessary.

You can find further details in the SAI about the payments made by MFD and its affiliates and the services provided by financial intermediaries. Financial intermediaries may charge you additional fees and/or commissions other than those disclosed in this prospectus. You should ask your financial intermediary for information about any payments it receives from MFD and its affiliates and any services it provides, as well as about fees and/or commissions it charges you. Financial intermediaries that sell fund shares may also act as a broker/dealer in connection with an MFS fund’s purchase or sale of portfolio securities. However, the fund and MFS do not consider financial intermediaries’ sales of shares of an MFS fund as a factor when choosing broker/dealers to effect portfolio transactions for the MFS funds.

How To Purchase, Redeem, and Exchange Shares

You may purchase, redeem, and exchange shares of the fund in the manner described below. The fund is generally only available to U.S. residents with a valid U.S. tax identification number (and to certain other qualified investors). If you buy or sell shares of a fund through a retirement account, Section 529 tuition plan, or financial intermediary, the procedures for buying, selling, and exchanging shares of the fund and the features, policies, and fees may differ from those discussed in this prospectus. Investment minimums may be waived or reduced for certain types of investors and investments and your financial intermediary may have additional minimums.

How to Purchase Shares

Your shares will be bought at the offering price (the net asset value per share plus any applicable initial sales charge) next calculated after your purchase order is received in proper form. Your financial intermediary is responsible for transmitting your purchase order to the fund in proper form and in a timely manner. MFSC reserves the right to reject any purchase order that is not in proper form. The specific requirements for proper form depend on the type of account and transaction and the method of purchase; contact MFSC if you have questions about your particular circumstances. Certain restrictions apply to the use of a transfer on death registration. You or your financial intermediary should contact MFSC to obtain a Transfer on Death registration form and for information regarding MFSC’s other requirements for transfer on death registrations. If payment for a purchase order is not received by the time required under applicable law, your purchase order may be cancelled and your financial intermediary could be liable for any costs or losses.

The fund may reject for any reason, or cancel as permitted or required by law, any purchase orders. The fund may stop offering shares completely, or may offer shares only on a limited basis, for a period of time or permanently.

The fund is required by law to obtain from you certain information that will be used to verify the identity of the account owner. If you do not provide the information, the fund will not be able to open your account. The fund must also take certain steps to verify that the account information you provide is correct.

Purchases of Class B shares are closed to new and existing investors except through reinvestment of dividends and capital gain distributions. Existing investors may continue to exchange their Class B shares for the same share class of another MFS fund.

Class A, Class B, and Class C Shares. You can establish an account by having your financial intermediary process your purchase or by contacting MFSC directly.

Purchases of Class B shares are subject to a total account value limitation at the time of purchase of $99,999, and purchases of Class C shares are subject to a total account value limitation at the time of purchase of $999,999. If your existing accounts for all share classes held with the MFS funds have a total value equal to $99,999 for Class B share purchases or $999,999 for Class C share purchases, you will not be able to purchase Class B or Class C shares, as applicable. For the purpose of determining your total account value, existing accounts for all share classes held with the MFS funds that are linked under a Letter of Intent or Right of Accumulation will be included.

The fund or its agents may at their discretion accept a purchase request for Class B shares or Class C shares that would otherwise exceed the total account value limitation of $99,999 and $999,999, respectively, under certain circumstances, including purchases by certain types of group or sponsored retirement plans.

Class I Shares. You can establish an account through your MFD representative, by having your financial intermediary process your purchase, or by contacting MFSC directly.

Class R1, Class R2, Class R3, Class R4, and Class R6 Shares. You can establish an account through your financial intermediary or by contacting MFSC directly. For Class R6 investors, you can also establish your account through your MFD representative.

Additional Purchases

You may purchase additional shares through your financial intermediary or MFSC.

Additional Purchases Directly Through MFSC.

Additional Purchases Through Your Financial Intermediary. You can have your financial intermediary purchase shares on your behalf. Your financial intermediary will be responsible for furnishing all necessary documents to MFSC and may charge you for this service.

How to Redeem Shares

Your shares will be sold at the net asset value per share next calculated after your redemption order is received in proper form, minus any applicable CDSC and/or other fees. Your financial intermediary is responsible for transmitting your redemption order

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to the fund in proper form and in a timely manner. MFSC reserves the right to reject any redemption request that is not in proper form. The specific requirements for proper form depend on the type of account and transaction and the method of redemption; contact MFSC if you have questions about your particular circumstances.

In certain circumstances, you will need to have your signature guaranteed and/or submit additional documentation to redeem your shares. In general, no signature guarantee is required for a redemption order for up to $100,000 that is signed by all owners or fiduciaries identified in the account registration, paid as registered, and mailed to the address of record. However, if you have changed your address of record within 30 days of your redemption order, a Medallion signature guarantee will generally be required. A Medallion signature guarantee will also generally be required if you want to send your redemption proceeds to a pre-designated bank account that was added or changed on your account within 30 days of your redemption order.

The fund typically processes your redemption proceeds, after your request is received in proper form, by the end of the next business day for payment by wire and payment processed through DTCC’s Defined Contribution Clearance & Settlement System or Fund/SERV system (financial intermediaries can choose to have payments processed through the Fund/SERV system by the end of the second business day instead of the next business day), and by the end of the second business day for payment by check and Automated Clearing House. However, the fund may delay processing your redemption request for up to seven days. Under unusual circumstances, such as when the NYSE is closed, trading on the NYSE is restricted, or as permitted by the SEC, the fund may suspend redemptions or postpone payment for more than seven days.

If you recently purchased shares and subsequently request a redemption, redemption proceeds (other than in an exchange) for the shares you recently purchased will generally be delayed until money from such purchases sufficient to cover your redemption has been received and collected. Receiving and collecting payment can take up to seven business days after a purchase. You can avoid this delay by purchasing shares by wire transfer.

The fund typically expects to meet redemption requests by holding cash and/or cash equivalents, selling investments, and using overdraft provisions or lines of credit. The fund may also meet redemption requests by redeeming shares in kind for large redemptions or during stressed market conditions. If, during any 90-day period, you redeem shares in an amount greater than the lesser of $250,000 or 1% of fund net assets, and if MFS determines it to be feasible and appropriate, the fund may pay the redemption amount above such threshold by a distribution in kind of portfolio securities (redemption in kind). In the event that the fund makes a redemption in kind, you should expect to incur brokerage and other transaction charges when converting the securities to cash, and the securities will likely increase or decrease in value before you sell them.

You may redeem your shares either by having your financial intermediary process your redemption or by contacting MFSC directly.

Redeeming Directly Through MFSC.

Redeeming Through Your Financial Intermediary. You can have your financial intermediary process a redemption on your behalf. Your financial intermediary will be responsible for furnishing all necessary documents to MFSC and may charge you for this service.

Signature Guarantee/Additional Documentation. If a Medallion signature guarantee is required, your signature may be guaranteed by an eligible bank, broker/dealer, credit union, national securities exchange, registered securities association, clearing agency, or savings association that participates in a Medallion program recognized by the Securities Transfer Association. MFSC may require additional documentation for certain types of registrations and under certain circumstances. Signature guarantees and this additional documentation shall be accepted in accordance with policies established by MFSC, and MFSC may, at its discretion, change and make certain exceptions to these requirements. Please contact MFSC with any questions and for the requirements for your particular situation.

Share Certificates. If certificates are outstanding for your shares, you may only redeem such shares by mailing the certificates to MFSC. Telephone, electronic, and systematic withdrawal plan redemptions and checkwriting are not available if certificates are outstanding for your shares.

Involuntary Redemptions. Because it is costly to maintain small accounts, the MFS funds have reserved the right to redeem your Class A, Class B, Class C, or Class I shares without your permission when your account contains less than $500 for any reason,

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including market fluctuation. Before the fund makes such a redemption, you will be notified and given 60 days to increase your investment to at least $500. Certain accounts are not subject to these provisions. Any applicable CDSC will be waived for shares redeemed due to the small size of a shareholder’s account.

In addition, the MFS funds have reserved the right to redeem your shares without your permission in cases of threatening conduct or suspicious, fraudulent, or illegal activity. Any applicable CDSC will be assessed upon redemption of your shares.

How to Exchange Shares

An exchange involves the redemption of shares of one fund and the purchase of shares of another fund.

Exchange Privilege. You can exchange your shares for shares of the same class, if available, of most other MFS funds at their respective net asset values by having your financial intermediary process your exchange request or by contacting MFSC directly.

You can exchange your Class A shares and your Class I shares for shares of MFS U.S. Government Money Market Fund, if available, according to the terms of such fund’s prospectus.

Shares otherwise subject to a CDSC will not be charged a CDSC in an exchange. Shares will retain the CDSC schedule in effect based upon a pro rata share of the CDSC from the exchanged fund and the original purchase date of the shares subject to the CDSC.

You should read the prospectus of the MFS fund into which you are exchanging and consider the differences in investment objectives, policies, and risks, and in fees and expenses, before making an exchange. The exchange privilege may be changed or discontinued at any time, and all exchanges are subject to certain limitations and the MFS funds’ policies concerning disruptive trading practices, which are designed to protect the funds and their shareholders from the harmful effects of frequent trading. No more than ten exchanges may be made in any one exchange request by telephone.

The MFS funds allow certain financial intermediaries to place exchange orders on behalf of a group of their discretionary investment advisory clients (“group exchange orders”). As with any exchange request, the funds and their agents reserve the right to reject any group exchange order, and the funds’ agents will generally reject any group exchange order received by the funds or their agents after 1 p.m., Eastern time. In addition, MFD has agreements with certain financial intermediaries which set forth the terms and conditions under which group exchange orders may be placed by these financial intermediaries. These conditions may be more restrictive than those applicable to individual exchange orders, and may include the requirement to provide the funds or their agents with advance notice of group exchange orders.

Automatic Exchange Plan (not available for Class R1, Class R2, Class R3, Class R4, and Class R6 shares). If you have a balance of at least $2,000 in your account in the fund, you may participate in the automatic exchange plan, a dollar-cost averaging program. This plan permits you to make automatic periodic exchanges from your account in the fund for shares of the same class of other MFS funds. Exchanges will generally be made at net asset value without any sales charges; however, you will pay the initial sales charge, if applicable, if you have not already paid this charge on these shares.

Disruptive Trading

General Purchase and Exchange Limitation Policy. The MFS funds reserve the right to restrict, reject, or cancel, without any prior notice, any purchase or exchange order, including transactions believed to represent frequent or other disruptive trading activity. In the event that MFSC rejects or cancels an exchange request, neither the redemption nor the purchase side of the exchange will be processed. Each MFS fund reserves the right to delay for one business day the processing of exchange requests in the event that, in MFSC’s judgment, in consultation with MFS, as appropriate, such delay would be in the fund’s best interest, in which case both the redemption and purchase side of the exchange will receive the funds’ net asset values at the conclusion of the delay period.

Disruptive Trading Risks. To the extent that the MFS funds or their agents are unable to curtail disruptive trading practices in a fund (e.g., frequent trading) or to the extent there are large or frequent redemptions in a fund, these purchases and/or redemptions can interfere with the efficient management of the fund’s portfolio, may result in increased transaction and administrative costs, and may adversely impact the fund’s performance.

In addition, to the extent that the fund invests in foreign securities, the interests of long-term shareholders may be diluted as a result of time-zone arbitrage, a short-term trading practice that seeks to exploit changes in the value of the fund’s investments that result from events occurring after the close of the foreign markets on which the investments trade, but prior to the time the fund determines its net asset value. The fund’s use of fair valuation can serve to reduce arbitrage opportunities available to short-term traders, but there is no assurance that the fund’s fair valuation policies and procedures will prevent dilution of the fund’s net asset value by short-term traders.

To the extent that the fund invests in securities that trade infrequently or are difficult to value, such as the securities of smaller companies, high yield debt instruments, and floating rate loans, the interests of long-term shareholders may be diluted as a result of price arbitrage, a short-term trading strategy that seeks to exploit perceived pricing inefficiencies in the fund’s investments. Such short-term trading strategies may interfere with efficient management of the fund’s portfolio to a greater degree than funds that invest in more frequently traded or liquid securities, in part because the fund may have difficulty selling these portfolio securities at advantageous times or prices to satisfy large and/or frequent redemption requests. Any successful price arbitrage may also cause dilution in the value of fund shares held by other shareholders.

Frequent Trading. The fund is not intended to serve as a vehicle for frequent trading. The Board of Trustees of the fund has adopted the purchase and exchange limitation policy described below, which it believes is reasonably designed to discourage frequent fund share transactions. MFSC seeks to monitor and enforce this policy, subject to oversight by the Board of Trustees. The fund may alter its policies at any time without notice to shareholders.

MFSC will generally restrict, reject or cancel purchase and exchange orders into the fund if MFSC determines that an accountholder has made two exchanges, each in an amount of $15,000 or more, out of an account in the fund during a calendar quarter (“two exchange limit”). This policy does not apply to Section 529 tuition plans and MFS money market funds, or to exchanges initiated by a retirement plan trustee or sponsor rather than by a plan participant, and other similar non-discretionary exchanges (e.g., in connection with fund mergers/acquisitions/liquidations). MFSC may make exceptions to this policy if, in its judgment, the transaction does not represent frequent trading activity, such as purchases made through systematic purchase plans (but not systematic exchange plans), payroll contributions, or distribution investment programs. In applying this policy, MFSC considers the information available to it at the time and reserves the right to consider trading effected through multiple accounts that are under common ownership, control, or influence to be trading out of a single account.

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Exchanges made on the same day in the same account are aggregated for purposes of counting the number and dollar amount of exchanges made by the accountholder (e.g., an accountholder who on the same day exchanges $16,000 from the fund into two other MFS funds, by exchanging $8,000 into each of the two MFS funds, will be viewed as having made one exchange transaction exceeding $15,000 in value).

Omnibus accounts, in which shares are held in the name of a financial intermediary on behalf of multiple underlying shareholders, are a common form of holding shares among certain financial intermediaries such as brokers, retirement plans, investment advisers, and insurance companies.

Financial intermediaries are required to reject any purchase or exchange orders in the fund if they believe the orders represent frequent trading activity unless they notify MFSC or an affiliate in writing that they do not monitor for frequent trading (“Waived Financial Intermediary”). With respect to Waived Financial Intermediaries, MFSC will take action reasonably designed to discourage frequent trading that is not in the best interests of the fund by the customers of such Waived Financial Intermediary, including requesting underlying shareholder account data more frequently than from other financial intermediaries.

With respect to omnibus accounts that provide MFSC with underlying shareholder data daily, MFSC will apply the two exchange limit directly to underlying shareholders. For all other accounts which MFSC determines are omnibus accounts, MFSC will apply the two exchange limit to the omnibus account itself. Applying the two exchange limit to the omnibus account itself will not necessarily detect violations of the two exchange limit by underlying shareholders. If the financial intermediary associated with an omnibus account that has triggered the two exchange limit demonstrates to MFSC, as determined in MFSC’s sole discretion, that no single underlying shareholder triggered the two exchange limit, then MFSC will remove any restrictions, rejections, or cancellations imposed on the account. If the financial intermediary demonstrates to MFSC that a single underlying shareholder triggered the two exchange limit and that the financial intermediary itself will apply applicable restrictions, rejections, or cancellations to that underlying shareholder, MFSC will likewise remove any restrictions, rejections, or cancellations imposed on the omnibus account. Otherwise, MFSC will continue to apply the two exchange limit to the omnibus account.

Certain financial intermediaries may use procedures to restrict frequent trading by their customers who invest in the fund while others may not employ any procedures to restrict frequent trading. Such procedures, if any, may be less restrictive than the fund’s purchase and exchange limitation policy, may permit transactions not permitted by the fund’s purchase and exchange limitation policy, including transactions in excess of the two exchange limit, and/or may prohibit transactions not subject to the fund’s purchase and exchange limitation policy. You should consult your financial intermediary about any restrictions it imposes on frequent trading.

There is no assurance that MFSC will be able to detect or prevent frequent trading. Shareholders seeking to engage in frequent trading practices may deploy a variety of strategies to avoid detection, and MFSC is generally not able to identify trading by a particular underlying shareholder within an omnibus account, which makes it difficult or impossible to determine if a particular underlying shareholder is engaged in frequent trading.

MFSC reviews trading activity to detect trading activity that may be indicative of frequent trading based on its internal parameters for detecting frequent trading, including reviewing transactions (including exchanges) that exceed a certain dollar amount, transactions involving similar dollar amounts, or transactions that occur close in time to other transactions in the same account or in multiple accounts that are under common ownership or influence. Any or all of these parameters (including those not listed) may change at any time. If MFSC detects suspicious trading activity at the omnibus account level, it will contact the financial intermediary to request underlying shareholder level activity to determine whether there is underlying shareholder level frequent trading. However, the underlying shareholder data received may not reflect information about the ultimate underlying shareholders because there may be multiple layers of omnibus accounts. If frequent trading is identified, MFSC will take appropriate action, such as requesting the financial intermediary to prohibit purchases into the account by the underlying shareholder, requiring purchases by the underlying shareholder to be submitted only by mail, or prohibiting purchases from the financial intermediary.

MFSC’s ability to monitor and deter frequent trading in omnibus accounts depends on, among other factors, the frequency with which MFSC requests underlying shareholder account data from omnibus accounts. MFSC expects to request underlying shareholder data based on its assessment of the likelihood of frequent trading by underlying shareholders among other factors. MFSC expects to request underlying shareholder data from Waived Financial Intermediaries more frequently than from other financial intermediaries. There is no assurance that MFSC will request data with sufficient frequency to detect or prevent frequent trading in omnibus accounts effectively.

Other Considerations

Unauthorized Transactions. MFS will not be responsible for losses that result from unauthorized transactions unless MFSC does not follow procedures reasonably designed to verify your identity. If an account has more than one owner or authorized person, MFSC will accept telephone and online instructions from any one owner or authorized person, except MFSC will require instructions for a redemption from all trustees of trust accounts registered with multiple trustees. It is important that you contact MFSC immediately about any transactions made through MFSC you believe to be unauthorized.

Ability to Contact MFSC. Certain methods of contacting MFSC, such as by mail, telephone, or electronically, may be unavailable or delayed (for example, after natural disasters, during periods of significant/major political, social, or economic instability, or in the event of a cyber security incident).

Unclaimed Accounts and Uncashed Checks. If your account has no activity for a certain period of time and/or mail sent to you on behalf of the fund is returned by the post office, the fund may be required to transfer your account and any assets related to uncashed checks to the appropriate state under the state’s abandoned property laws. To avoid such action, it is important to keep your account address up to date and periodically communicate with the fund, MFSC, or your financial intermediary. When a traditional IRA account is transferred to a state as abandoned property, the Internal Revenue Service considers the transfer a taxable distribution subject to federal income tax withholding and reporting. The fund and MFSC will not be liable to shareholders or their representatives for good faith compliance with abandoned property laws.

Reservation of Other Rights. In addition to the rights expressly stated elsewhere in this prospectus, subject to applicable rules, laws, and regulations, MFSC reserves the right to: 1) alter, add, or discontinue any conditions of purchase, redemption, service, or privilege at any time without notice; 2) freeze any account or suspend account services when MFSC has received reasonable notice (written or otherwise) of a dispute between registered or beneficial account owners or when MFSC believes a fraudulent

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transaction may occur or has occurred; and 3) change, impose, discontinue, or waive any fee it charges.

Anti-Money Laundering Restrictions. Federal law requires the fund to implement policies and procedures reasonably designed to prevent, detect, and report money laundering and other illegal activity. The fund, consistent with applicable federal law, may redeem your shares and close your account; suspend, restrict, or cancel purchase and redemption orders; process redemption requests and withhold your proceeds; and take other action if it is unable to verify your identity within a reasonable time or conduct required due diligence on your account or as otherwise permitted by its anti-money laundering policies and procedures. Any applicable CDSC will be assessed upon redemption of your shares.

Confirmations in Quarterly Statements. Transactions made under certain periodic investment and withdrawal programs (including reinvestment plans) will be confirmed on quarterly account statements.

Other Information

Valuation

The price of each class of the fund’s shares is based on its net asset value. The net asset value of each class of shares is determined each day the NYSE is open for trading as of the close of regular trading on the NYSE (generally 4:00 p.m. Eastern time). However, net asset value may be calculated earlier in emergency situations or as otherwise permitted by the SEC. Net asset value per share is computed by dividing the net assets allocated to each share class by the number of shares outstanding for that class. On days when the NYSE is closed (such as weekends and holidays), net asset value is not calculated, and the fund does not transact purchase and redemption orders. To the extent the fund’s assets are traded in other markets on days when the fund does not price its shares, the value of the fund’s assets will likely change when you will not be able to purchase or redeem shares of the fund.

To determine net asset value, the fund’s investments for which reliable market quotations are readily available are valued at market value, and funds in which the fund invests are generally valued at their net asset value per share. Certain short term debt instruments may be valued at amortized cost.

The Board of Trustees has delegated primary responsibility for determining or causing to be determined the value of the fund’s investments to MFS pursuant to the fund’s valuation policy and procedures approved by the Board of Trustees. The Board of Trustees has designated MFS as the “valuation designee” of the fund. As valuation designee, MFS will determine the fair value, in good faith, of securities and other instruments held by the fund for which market quotations are not readily available and, among other things, will assess and manage material risks associated with fair value determinations, select, apply, and test fair value methodologies, and oversee and evaluate pricing services used in valuing the fund’s investments. If MFS determines that reliable market quotations are not readily available, investments are valued at fair value as determined in good faith by MFS in accordance with the fair valuation policy and procedures adopted by MFS.

In addition, investments may be valued at fair value if MFS determines that an investment’s value has been materially affected by events occurring after the close of the exchange or market on which the investment is principally traded (such as a foreign exchange or market) and prior to the determination of the fund’s net asset value, or after the halting of trading of a specific security where trading does not resume prior to the close of the exchange or market on which the security is principally traded. Events that occur on a frequent basis after foreign markets close (such as developments in foreign markets and significant movements in the U.S. markets) and prior to the determination of the fund’s net asset value may be deemed to have a material effect on the value of securities traded in foreign markets. Accordingly, foreign equity securities may often be valued at fair value. MFS generally relies on third-party pricing services or other information (such as the correlation with price movements of similar securities in the same or other markets; the type, cost, and investment characteristics of the security; the business and financial condition of the issuer; and trading and other market data) to assist in determining whether to fair value and at what value to fair value an investment.

The value of an investment for purposes of calculating the fund’s net asset value can differ depending on the source and method used to determine value. When fair valuation is used, the value of an investment used to determine the fund’s net asset value may differ from quoted or published prices for the same investment. There can be no assurance that the fund could obtain the fair value assigned to an investment if it were to sell the investment at the same time at which the fund determines its net asset value per share.

Distributions

The fund intends to declare and pay any dividends to shareholders at least annually.

Any capital gains are distributed at least annually.

Distribution Options

The following distribution options are generally available for Class A, Class B, Class C, and Class I shares:

Not all options are available for every class or every account.

Dividends and capital gain distributions for Class R1, Class R2, Class R3, Class R4, and Class R6 shares can only be reinvested in additional shares of the fund, except with respect to Class R6 shares, (i) certain institutional shareholders, (ii) shareholders who purchase shares through brokerage programs of certain financial intermediaries, and (iii) shareholders who purchase shares through asset-based fee programs of certain financial intermediaries may elect any of the other distribution options described above.

The distribution option for accounts with dividend distributions of less than $10 generally will be changed to reinvestment in additional shares of the fund. If you have elected to receive distributions in cash, and the postal service is unable to deliver checks to your address of record, or you do not respond to mailings from MFSC with regard to uncashed distribution checks, your distribution option may be converted to having all distributions reinvested in additional shares. You should contact MFSC to change your distribution option, and your request to do so must be received by MFSC before the record date for a distribution in order to be effective for that distribution. No interest will accrue on amounts represented by uncashed distribution checks.

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Tax Considerations

The following discussion is very general. You are urged to consult your tax adviser regarding the effect that an investment in the fund will have on your particular tax situation, including possible foreign, state, and local taxes. This discussion does not apply to shares of the fund held through tax-advantaged accounts or by shareholders that are not “U.S. persons” under the Internal Revenue Code of 1986, as amended.

The fund expects to distribute substantially all of its income and gains annually. Distributions from the fund are taxable whether you receive them in cash or reinvest them in additional shares. If you buy shares when the fund has realized but not yet distributed ordinary income or capital gains, you will pay full price for the shares and then receive a portion back as a taxable distribution.

Any gain resulting from the redemption, sale, or exchange of your shares will generally also be subject to tax.

For federal income tax purposes, distributions of investment income are generally taxable as ordinary income. Taxes on distributions of capital gains are determined by how long the fund owned or is deemed to have owned the investments that generated them, rather than how long you have owned your shares. Distributions of gains from the sale of investments that the fund owned for more than one year and that are properly reported by the fund as capital gain dividends will be treated as long-term capital gains includible in net capital gain and taxed to individuals at reduced rates. Distributions of gains from the sale of investments that the fund owned for one year or less will be taxable as ordinary income. If some or all of the fund’s income derives from “qualified dividend income” and if you are an individual who meets holding period and other requirements with respect to the fund’s shares, those distributions that are properly reported by the fund as derived from qualified dividend income are taxed to you at the reduced rates applicable to net capital gains.

A 3.8% Medicare contribution tax is generally imposed on distributions paid by the fund (other than exempt-interest dividends, if any) to, and net gains recognized on the sale, redemption, exchange, or other taxable disposition of shares of the fund by certain individuals, estates, and trusts to the extent their income exceeds certain threshold amounts.

The fund’s investments in foreign securities may be subject to foreign withholding or other taxes, which will decrease the fund’s return on those securities; you generally are not expected to be eligible to claim a credit (or a deduction) for such amounts on your federal income tax return.

The Form 1099 that is provided to you early each year details your distributions from the fund and how they are treated for federal tax purposes, and cost basis information for shares purchased on or after January 1, 2012, that you redeemed, sold, or exchanged. Our default method for calculating cost basis is average cost basis. More information about other cost basis methods and a cost basis selection/change form are available on mfs.com. If your account is held by your financial intermediary, you must contact your financial intermediary to obtain information about available cost basis methods and cost basis elections for your account.

Information about the Summary Prospectus, Prospectus, and SAI

The summary prospectus, prospectus, and SAI for a fund provide information concerning the fund. The summary prospectus, prospectus, and SAI are updated at least annually and any information provided in a summary prospectus, prospectus, or SAI can be changed without a shareholder vote unless specifically stated otherwise. The summary prospectus, prospectus, and SAI are not contracts between the fund and its shareholders and do not give rise to any contractual rights or obligations or any shareholder rights other than any rights conferred explicitly by federal or state securities laws that may not be waived.

Provision of Financial Reports and Summary Prospectuses

The fund produces financial reports every six months and updates its summary prospectus and prospectus annually. To avoid sending duplicate copies of materials to households, only one paper copy of the fund’s annual and semiannual report and summary prospectus may be mailed to shareholders having the same last name and residential address on the fund’s records. However, any shareholder may contact MFSC (please see back cover for address and telephone number) to request that copies of these reports and summary prospectuses be sent personally to that shareholder.

Additional Information on Fees and Expenses and Performance

Fees and Expenses

The annual fund operating expenses shown in “Fees and Expenses” are based on annualized expenses reported during the fund’s most recently completed fiscal year expressed as a percentage of a class’ average net assets during the period. Annual fund operating expenses have not been adjusted to reflect the fund’s current asset size. In general, annual fund operating expenses, expressed as a percentage of a class’ average net assets, increase as the fund’s assets decrease. Annual fund operating expenses will likely vary from year to year.

Performance Information

All performance information shown in the “Class A Bar Chart” and the “Performance Table” reflects any applicable fee reductions and waivers and expense reimbursements in effect during the periods shown; without these, the performance would have been lower.

From time to time, the fund may receive proceeds from litigation settlements, without which performance would be lower.

The fund commenced operations on January 2, 1996, with the offering of Class A shares, and subsequently offered Class B, Class C, and Class I shares on January 2, 1997, Class R2 shares on October 31, 2003, Class R1, Class R3, and Class R4 shares on April 1, 2005, and Class R6 shares on January 2, 2013.

In the “Performance Table,” performance for Class R6 shares includes the performance of the fund’s Class I shares, adjusted to take into account differences in sales loads and class specific operating expenses (such as Rule 12b-1 fees), if any, for periods prior to their offering.

MFS Core Equity Fund

Financial Highlights

The financial highlights are intended to help you understand a fund’s financial performance for the past five years. Certain information reflects financial results for a single fund share. The total returns in the financial highlights represent the rate by which an investor would have earned (or lost) on an investment in a fund (assuming reinvestment of all distributions) held for the entire period. This information has been audited by the fund’s independent registered public accounting firm, whose report, together with the fund’s financial statements, is included in the fund’s Annual Report to shareholders. The fund’s Annual Report is available upon request by contacting MFSC (please see back cover for address and telephone number). The fund’s independent registered public accounting firm is Ernst & Young LLP.

Class A

Class B 

MFS Core Equity Fund 

Class C

Class I

MFS Core Equity Fund 

Class R1