The relationship of obesity to environmental factors such as season, region, and population density was examined in children studied during cycle II of the National Health Examination Survey. This survey selected 7119 children aged 6 to 11 yr old from a representative noninstitutionalized sample of the United States population. The prevalence of obesity (triceps skinfold more than the 85th percentile) was significantly more in the Northeast and Midwest than in the West and significantly more in large metropolitan areas than in areas with lower population densities. The prevalence of obesity was generally lowest in the summer and highest in the fall or winter. Each environmental variable was associated with 2- to 3-fold variations in the prevalence of obesity. The effect of region, population density, and season appeared independent of race and socioeconomic status. Similar relationships were found for superobesity (triceps skinfold more than the 95th percentile). These results emphasize that environmental variables significantly affect the prevalence of obesity and could help account for the wide variations in prevalence that have been previously published. Understanding the mechanisms by which the environment affects childhood obesity may improve the effectiveness of community level interventions.