Assortative mating for obesity may contribute genetically to the obesity epidemic by increasing the risk in subsequent generations. To test the hypothesis that mate selection is promoted by similarity in body fatness and may thus contribute to the current obesity epidemic, validated questionnaire data on family members of 7834 obese probands and 829 subjects from a random selection of the general Swedish population were analyzed. Obesity concordance in parents was associated with an obesity prevalence of 20.1% in adult offspring compared with 8.2% if parents were obese discordant, and 1.4% if parents were non obese concordant. Therefore, assortative mating for obesity appears to confer a higher risk of obesity in the offspring. As a conclusion, the authors recommend that parental obesity concordance be considered as an easily identifiable genetic risk factor for designing primary prevention programs.