This paper examines problems in the CRSP Survivor Bias Free U.S. Mutual Fund Database (CRSP, 1998) and compares returns contained in it to those in Morningstar. The CRSP database has an omission bias that has the same effects as survivorship bias. Although all mutual funds are listed in CRSP, return data is missing for many and the characteristics of these funds differ from the populations. The CRSP return data is biased upward and merger months are inaccurately recorded about half the time. Differences in returns in Morningstar and CRSP are a problem for older data and small funds.