Accruing evidence indicates that mate selection is promoted by similarity in body fatness. Assortative mating for obesity may contribute genetically to the obesity epidemic by increasing the risk in subsequent generations. To test this hypothesis, the authors analyzed measured and validated questionnaire data on family members, obtained between 1987 and 2000 from 7,834 obese probands and from 829 subjects randomly ascertained from the general Swedish population. Spouse correlations in body mass index were strongest among couples with the shortest duration of cohabitation. Obesity concordance in parents was associated with an obesity prevalence of 20.1% in adult offspring compared with 1.4% if parents were concordantly nonobese (odds ratio = 18.3, 95% confidence interval: 9.0, 37.4). The prevalence was 8.2% if parents were obesity discordant (odds ratio = 6.5, 95% confidence interval: 3.2, 13.2). No association was found between rearing parents' and nonbiologic offspring's body mass index. These results agree with the hypothesis that assortative mating for obesity confers a higher risk of obesity in the offspring generation and thus contributes to the obesity epidemic. Parental obesity concordance is a strong, easily identifiable genetic risk factor that should be considered in the complex network of risk factors for obesity in designing primary prevention programs.