Participation in the Supplemental Nutrition Assistance Program (SNAP) reached an all-time high of 40.2 million persons in March 2010, which means the program affects a substantial fraction of Americans. A significant body of research has emerged suggesting that participation in SNAP increases the probability of being obese for adult women and has little effect on the probability for adult men. However, studies addressing the effects of participation on children have produced mixed results. This paper examines the effect of long-term SNAP participation on the Body Mass Index (BMI) percentile and probability of being overweight or obese for children ages 5–18 using data from the National Longitudinal Survey of Youth 1979 Children and Young Adults data set. An instrumental variables identification strategy that exploits exogenous variation in state-level program parameters, as well as state and federal expansions of the Earned Income Tax Credit (EITC), is used to address the endogeneity between SNAP participation and obesity. SNAP participation is found to significantly reduce BMI percentile and the probability of being overweight or obese for boys and girls ages 5–11 and boys ages 12–18. For girls ages 12–18, SNAP participation appears to have no significant effect on these outcomes. Copyright © 2011 John Wiley & Sons, Ltd.