Background: Community-based studies have suggested a multigenerational pattern of obesity affecting children's risk of overweight, but no national data have substantiated such a pattern. Our objective was to examine the prevalence of overweight [body mass index (BMI) [≥]95th percentile for age and sex] among children aged 5 to 19 in a national sample, stratified by the obesity status of their parents and grandparents. Methods: We used a secondary analysis of the Panel Study of Income Dynamics, Child Development Supplement, a multigenerational, genealogical, prospective cohort study of the US population. Self-report height and weight data from adults and measured height and weight data for children were used to calculate BMI. The prevalence of child overweight was calculated for different possible combinations of parental and grandparental BMI status, including missing status. Results: The sample included 2591 children aged 5 to 19 years, for whom parental BMI data were available for 94% and grandparental BMI data were available for 61%. Prevalence of childhood overweight (18.6%) in the sample was comparable with contemporaneous measured national data from other sources. Among children with normal-weight parents and normal-weight grandparents, 7.9% were overweight. In contrast, among children with overweight parents (BMI 25-29.9) and normal-weight grandparents, 17.9% were overweight, and among children with obese parents (BMI [≥]30) and normal-weight grandparents, 31.9% were overweight (P < .0001). Importantly, when parents were normal weight, if grandparents were obese, then the prevalence of child overweight was 17.4% (P < .0001). The prevalence of child overweight was similarly elevated (16.4%) when parents were normal weight and grandparental BMI was missing. Conclusions: This is the first national study to find an association of child weight status with grandparental obesity, distinct from parental obesity. Primary care physicians may find it helpful to consider grandparents' weight status in judging risk of childhood overweight for their patients, especially when parents' weight is normal.