OBJECTIVE: To study associations between number of children and obesity in middle-aged women and men. METHODS: In the Health and Retirement Study, a national survey of households, we tested the association between increasing number of children and obesity (body mass index [BMI] >or= 30) in 9046 middle-aged women and men (4523 couples). RESULTS: Women (n = 4523) who were obese were more frequently nonwhite, reported lower household income, were more frequently employed outside the home, were less frequently covered by health insurance, and were more frequently less educated compared with nonobese women. Men (n = 4523) who were obese were younger, were more frequently African American, and were more frequently less educated and poorer compared with nonobese men. Among women, a 7% increase in risk of obesity was noted for each additional child, adjusting for age, race, household income, work status, physical activity, tobacco use, and alcohol use. Among men, a 4% increase in risk of obesity was noted for each additional child, adjusting for the same covariates. These sex differences were not significantly different. CONCLUSIONS: Previous research has demonstrated an association between number of children and obesity among women. These results suggest a similar association among men. Public health interventions focused on obesity prevention should target both parents, especially those parents with several children.