Open-end equity funds provide a diversified equity positions with little direct cost to investors for liquidity. This study documents a statistically significant indirect cost in the form of a negative relation between a fund's abnormal return and investor flows. Controlling for this indirect cost of liquidity changes the average fund's abnormal return (net of expenses) from a statistically significant −1.6% per year to a statistically insignificant −0.2% and also fully explains the negative market-timing performance found in this and other studies of mutual fund returns. Thus, the common finding of negative return performance at open-end mutual funds is attributable to the costs of liquidity-motivated trading.