The rising rate of obesity has reached epidemic proportions and is now one of the most serious public health challenges facing the US. However, the underlying causes for this increase are unclear. This paper examines the effect of family income changes on body mass index (BMI) and obesity using data from the National Longitudinal Survey of Youth 1979 cohort. It does so by using exogenous variation in family income in a sample of low-income women and men. This exogenous variation is obtained from the correlation of their family income with the generosity of state and federal Earned Income Tax Credit (EITC) program benefits. Income is found to significantly raise the BMI and probability of being obese for women with EITC-eligible earnings, and have no appreciable effect for men with EITC-eligible earnings. The results imply that the increase in real family income from 1990 to 2002 explains between 10 and 21% of the increase in sample women's BMI and between 23 and 29% of their increased obesity prevalence. (c) 2009 John Wiley & Sons, Ltd.