How much has food abundance, attributable to U.S. public agricultural R&D, contributed to the high and rising U.S. obesity rates? In this paper we investigate the effects of public investment in agricultural R&D on food prices, per capita calorie consumption, adult body weight, obesity, public health-care expenditures related to obesity, and social welfare. First we use an econometric model to estimate the average effect of an incremental investment in agricultural R&D on the farm prices of ten categories of farm commodities. Next, we use the econometric results in a simulation model to estimate the implied changes in prices and quantities consumed of nine categories of food for given changes in research expenditures. Finally, we estimate the corresponding changes in social welfare, including both the traditional measures of changes in economic surplus in markets for food and farm commodities, and changes in public health-care expenditures associated with the predicted changes in food consumption and hence obesity. We find that a 10 percent increase in the stream of annual U.S. public investment in agricultural R&D in the latter half of the 20th century would have caused a modest increase in average daily calorie consumption of American adults, resulting in small increases in social costs of obesity. On the other hand, such an increase in spending would have generated very substantial net national benefits given the very large benefit-cost ratios for agricultural R&D.