Once considered as a serious public health issue only in developed countries, now overweight and obesity have dramatically increased in low- and middle-income countries, especially in urban settings (WHO, 2008). The main purpose of this study is to explore the economic incentives for this rapid growth in obesity rates, by studying variations in obesity over time and across geographic regions in the United States. Although a number of researchers and policymakers have devoted significant resources to address the recent rapid rise in obesity in the United States, “the prevalence of overweight and obesity has increased sharply since the mid 1970s” (Centers for Disease Control, 2008) and most of this increase occurred in the 1980s and 1990s (Cutler, et al., 2003). More importantly, changes in food prices have also occurred over the past 30 years and have occurred simultaneously with the obesity epidemic (Finkelstein, et al., 2005). In this study, we investigate how the decline in food prices in the last three decades affects the long-run growth of obesity rates. We take the advantage of the large panel data that cover for the time periods with the fastest growth of obesity rates, by using metropolitan samples from the National Health Interview Survey (NHIS) and information on prices of food at home and food away from home from these major metropolitan areas for years 1976 to 2001. Specifically, instead of using absolute food prices, we explore the impacts from changes in relative prices of food at home and food away from home (i.e. food prices relative to prices for a market basket of consumer goods and services in these metropolitan areas), as well as changes in prices of food at home and food away from home on the growth in obesity rates during this time frame. We also control for the changes in contextual factors and changes in value of female in these metropolitan areas. Our findings reveal the important fact that changes in relative food prices can explain about 20 percent of the obesity growth during this time period and such effect is more pronounced for the low-educated. The results of the study provide an interpretation of the long-run growth of obesity rates in urban settings.