In an effort to ascertain the prevalence of obesity in young children, 48 nursery schools in Manhattan and Queens, New York City were surveyed. The weights and heights of 2606 children, ages 3 to 6 yr, were obtained. Three-hundred seventeen children (12.2%) were above 120% of ideal body weight and 122 (4.7%) were above 130% of ideal weight. Twenty percent of the children from schools attended by youngsters from low income families, 12% from schools attended by pupils from middle income families, and 5% from schools with pupils from families with a high income were classified as obese. Of the obese children's actual body weights 90% were at or greater than the 75th percentile for age while mean higher percentiles of the obese and nonobese children were not significantly different. Sex specific prevalence rates were similar for boys and girls. These data indicate that obesity is already prevalent in the nursery school population and that such obese children can be easily identified from weight and height measurements at a time when programs for successful permanent weight reduction and maintenance may be feasible.