Several social factors have been closely linked to obesity and thinness in adults. This study, based on 3,344 measurements of triceps skin-fold thickness found similar relationships in white urban children. Obesity was far more prevalent in the lower-class girls than in those of the upper class—nine times as prevalent by age 6. Similar though less striking differences were found between boys of upper and lower socioeconomic status. The pattern of thinness among girls was similar to that previously reported in women, with significantly more thinness in the upper-class group. Among boys, as among men, there were no such differences. The remarkably early onset of class-linked differences in prevalence of obesity underlines the importance of attempts to prevent the disorder in childhood. These attempts should be directed particularly toward those at high risk because of their lower socioeconomic status.