We test whether forecast bias affects individual investors’ stock trading by combining bias measures from laboratory experiments with administrative trade-level data. On average, subjects exhibit positive forecast bias (extrapolators), while a large minority exhibit negative bias (contrarians). Forecast bias is positively associated with past excess returns of purchased stocks: Extrapolators (contrarians) purchase past winners (losers). Forecast bias is negatively associated with capital gains of sold stocks. Forecast bias explains investor heterogeneity in the relation between market returns and net flows to stocks. Our study shows that forecast bias links past returns to trading decisions for purchases, sales, and net flows.