We assessed the association between weight, exercise, and stress and the duration of menstrual bleeding in a 1-year prospective menstrual diary study of 166 college first-year women, age 17-19 years. Low weight-for-height increased expected bleed duration by 0.39 day. Dieting to lose weight reduced bleed length by 0.43 day. Women who did no moderate or hard exercise bled about a quarter of a day longer than women at the median level of physical activity.