Research attributes much of the rise in obesity to technological progress reducing the cost of food consumption. We examine this hypothesis in the context of Walmart Supercenters, whose advancements in retail logistics have translated to substantial reductions in food prices. Using data from the Behavioral Risk Factor Surveillance System matched with Walmart Supercenter entry dates and locations, we examine the effects of Supercenters on body mass index (BMI) and obesity. We account for the endogeneity of Walmart Supercenter locations with an instrumental variables approach that exploits the unique geographical pattern of Supercenter expansion around Walmart’s headquarters in Bentonville, Arkansas. An additional Supercenter per 100,000 residents increases average BMI by 0.25 units and the obesity rate by 2.4 percentage points. These results imply that the proliferation of Walmart Supercenters explains 11% of the rise in obesity since the late 1980s, but the resulting increase in medical expenditures offsets only a small portion of consumers’ savings from shopping at Supercenters.