We use data from the 1980 to 2004 waves of the National Longitudinal Survey of Youth, 1979 Cohort to examine the association between the ethnic density of metropolitan areas and obesity among U.S. blacks and Latinos. Although minority groups’ obesity rates tend to be higher in metropolitan areas containing many co-ethnics, controlling for other areal characteristics and unobserved time-constant confounders via fixed-effects models dramatically alters this association. In the fixed-effects models, higher levels of co-ethnic density are inversely associated with black males’ obesity risk and unrelated to the obesity risk of black females, Latinas, and Latino males. For most groups, marrying and having children increases the risk of obesity.