The doubling of the prevalence of obesity in the U.S. since 1980 has generated tremendous interest in understanding the causes of obesity and its recent rise. We study one important potential cause that has been little investigated: U.S. agriculture policy. We document that, by pursuing policies that benefit agricultural producers, the U.S. promotes excess supply and lower prices which contribute to higher calorie intake and obesity. We estimate that agricultural subsidies account for 0.75 - 1.2 percent of the rise in average body mass index (BMI) between 1984 and 1999 in the U.S. An appreciation for how U.S. farm policy indirectly affects calorie intake and obesity may yield insights into how to best counter such unintended consequences and limit or reverse the recent rise in U.S. obesity.