The prevalence of overweight is higher for Hispanic children of immigrants than children of natives. This does not fit the pattern of the epidemiological paradox; the widely supported finding that immigrants tend to be healthier than their U.S.-born peers, and it suggests that exposure to the U.S. increases immigrant children’s risk of overweight. This study’s primary contribution is to better assess how exposure to the U.S. environment affects childhood overweight among a homogamous ethnic group, Mexican-Americans. We do so by using an innovative binational study design to compare the weight of Mexican-American children of immigrants, Mexican-American children of natives, and Mexican children in Mexico with different propensities of having immigrant parents. Cross-sectional data are derived from a pooled sample of 9,982 6–19 year old children living in either Mexico or the United States in the early 2000s. Mexican-resident children with a very high propensity to have immigrant parents have significantly lower percentile BMIs and lower odds of overweight than Mexican children with lower propensities of emigration and U.S.-resident Mexican-American children. This suggests that selection into immigration streams does not account for the high prevalence of overweight among children of Mexican immigrants. Rather, U.S. exposure significantly raises children of Mexican immigrants’ risk of being overweight. Moreover, second generation children have the highest percentile BMIs and greatest odds of overweight of all comparison groups, including children of natives. This suggests that they experience risks above and beyond the effects of exposure to American society.