OBJECTIVES: We examined the effect of physical education instruction time on body mass index (BMI) change in elementary school. METHODS: We examined data from a national sample of 9751 kindergartners in the United States who were reported on for 2 years. We used a difference-in-differences approach to examine the effect of an increase in physical education instruction time between kindergarten and first grade on the difference in BMI change in the 2 grades, using the same child as the control. RESULTS: One additional hour of physical education in first grade compared with the time allowed for physical education in kindergarten reduces BMI among girls who were overweight or at risk for overweight in kindergarten (coefficient = -0.31, P <.001) but has no significant effect among overweight or at-risk-for-overweight boys (coefficient = -0.07, P =.25) or among boys (coefficient = 0.04, P =.31) or girls (coefficient = 0.01, P =.80) with a normal BMI. CONCLUSIONS: Expanding physical education programs in schools, in the form in which they currently exist, may be an effective intervention for combating obesity in the early years, especially among girls.