ANNUAL REPORT December 31, 2020 COLUMBIA ACORN FAMILY OF FUNDS Managed by Columbia Wanger Asset Management, LLC Columbia Columbia Acorn® Fund Acorn International SelectSM Columbia Columbia Acorn International® Thermostat FundSM Columbia Columbia Acorn USA® Acorn European FundSM Beginning on January 1, 2021, as permitted by regulations adopted by the Securities and Exchange Commission, paper copies of the Funds’ annual and semiannual shareholder reports are no longer sent by mail, unless you specifically request paper copies of the reports. Instead, the reports are made available on the Funds’ website (columbiathreadneedleus.com/investor/), and each time a report is posted you will be notified by mail and provided with a website address to access the report. If you have already elected to receive shareholder reports electronically, you will not be affected by this change and you need not take any action. You may elect to receive shareholder reports and other communications from the Funds electronically at any time by contacting your financial intermediary (such as a broker-dealer or bank) or, for Fund shares held directly with the Funds, by calling 800.345.6611 or by enrolling in “eDelivery” by logging into your account at columbiathreadneedleus.com/investor/. You may elect to receive all future shareholder reports in paper free of charge. If you invest through a financial intermediary, you can contact your financial intermediary to request that you continue receiving paper copies of your shareholder reports. If you invest directly with the Funds, you can call 800.345.6611 to let the Funds know you wish to continue receiving paper copies of your shareholder reports. Your election to receive paper reports will apply to all Columbia Funds, including the Columbia Acorn Funds, held in your account if you invest through a financial intermediary or all Columbia Funds, including the Columbia Acorn Funds, held with the fund complex if you invest directly with the Funds. Not Federally Insured • No Financial Institution Guarantee • May Lose Value TABLE OF CONTENTS Columbia Acorn® Fund Fund at a Glance ........................................................................................................................................................ 3 Manager Discussion of Fund Performance........................................................................................................................ 5 Columbia Acorn International® Fund at a Glance ........................................................................................................................................................ 7 Manager Discussion of Fund Performance........................................................................................................................ 9 Columbia Acorn USA® Fund at a Glance ........................................................................................................................................................ 11 Manager Discussion of Fund Performance........................................................................................................................ 13 Columbia Acorn International SelectSM Fund at a Glance ........................................................................................................................................................ 15 Manager Discussion of Fund Performance........................................................................................................................ 17 Columbia Thermostat FundSM Fund at a Glance ........................................................................................................................................................ 19 Manager Discussion of Fund Performance........................................................................................................................ 21 Columbia Acorn European FundSM Fund at a Glance ........................................................................................................................................................ 23 Manager Discussion of Fund Performance........................................................................................................................ 25 Understanding Your Fund’s Expenses ............................................................................................................................. 27 Portfolio of Investments ............................................................................................................................................... 30 Statement of Assets and Liabilities ................................................................................................................................53 Statement of Operations .............................................................................................................................................. 57 Statement of Changes in Net Assets .............................................................................................................................. 59 Financial Highlights..................................................................................................................................................... 66 Notes to Financial Statements ...................................................................................................................................... 90 Report of Independent Registered Public Accounting Firm....................................................................................................111 Federal Income Tax Information .....................................................................................................................................112 Board of Trustees and Management of the Columbia Acorn Family of Funds ............................................................................113 Additional Information..................................................................................................................................................116 Columbia Acorn Family of Funds | Annual Report 2020 FUND AT A GLANCE Columbia Acorn® Fund Investment objective Columbia Acorn® Fund (the Fund) seeks long-term capital appreciation. Average annual total returns (%) (for the period ended December 31, 2020) Portfolio management Inception 1 Year 5 Years 10 Years Life of Fund Class A Excluding sales charges 10/16/00 29.18 16.26 11.83 14.08 Matthew A. Litfin, CFA Lead Portfolio Manager or Co-Portfolio Manager Including sales charges 21.80 14.89 11.17 13.94 since 2016 Advisor Class 11/08/12 29.44 16.54 12.10 14.43 Service with Fund since 2015 Class C Excluding sales charges 10/16/00 28.27 15.39 11.01 13.21 Including sales charges 27.30 15.39 11.01 13.21 Erika K. Maschmeyer, CFA Institutional Class 06/10/70 29.51 16.56 12.14 14.44 Co-Portfolio Manager since 2019 Institutional 2 Class 11/08/12 29.50 16.59 12.16 14.44 Service with Fund since 2016 Institutional 3 Class 11/08/12 29.54 16.65 12.21 14.45 Russell 2500 Growth Index 40.47 18.68 15.00 - Richard Watson, CFA Co-Portfolio Manager since 2019 Performance numbers reflect all Fund expenses. Returns for Class A shares are shown with and without the Service with Fund since 2006 maximum initial sales charge of 5.75%. Returns for Class C shares are shown with and without the maximum contingent deferred sales charge of 1.00% for the first year after purchase. The Fund’s other share classes are not TM subject to sales charges, but may be subject to other fees and expenses, and have limited eligibility. Please see Morningstar style box the Fund’s prospectus for details. Performance for different share classes will vary based on differences in sales charges and certain fees and expenses associated with each share class. Equity Style As stated in the May 1, 2020 prospectus (as supplemented), the Fund’s annual operating expense ratio is 0.86% Value Blend Growth for Institutional Class shares and 1.11% for Class A shares. All results shown assume reinvestment of distributions during the period. Returns do not reflect the deduction of taxes that a shareholder may pay on Fund distributions or on the redemption of Fund shares. Performance results Large reflect the effect of fee waivers or reimbursements of Fund expenses by the investment manager and/or its affiliates. Absent these fee waivers and/or expense reimbursement arrangements, performance results may have Size been lower. Medium The performance information shown represents past performance and is not a guarantee of future results. The investment return and principal value of your investment will fluctuate so that shares, when redeemed, may be Small worth more or less than their original cost. Current performance may be lower or higher than the performance data shown. You may obtain performance information current to the most recent month-end by contacting your financial The Morningstar Style Box is based on a fund’s intermediary, visiting columbiathreadneedleus.com/investor/ or calling 800.922.6769. portfolio holdings. For equity funds, the vertical axis shows the market capitalization of the stocks owned, The returns shown for periods prior to the share class inception date (including returns for the Life of the Fund, if shown, and the horizontal axis shows investment style which are since Fund inception) include the returns of the Fund’s oldest share class. These returns are adjusted to reflect (value, blend, or growth). Information shown is any higher class-related operating expenses of the newer share classes,