Recent evidence reveals significant negative serial correlation in aggregate (market-wide) stock returns. We extend this result to relative (market-adjusted) returns, demonstrating negative serial correlation in five-year returns. We then test two competing explanations: (1) market mispricing and (2) changing expected returns in an efficient market. The tests are conducted using the capital asset pricing model to estimate relative returns. The evidence suggests that negative serial correlation in relative returns is due almost entirely to variation in relative risks, and therefore expected relative returns, through time. We document substantial relative risk shifts, particularly for extreme-performing stocks.