Obesity increased monotonically from 1.2% to 3.8% of males age 17 (1967-2003). Low socioeconomic status had an independent positive effect on obesity. The likelihood of obesity had risen more steeply over time among the low socioeconomic status group than among other adolescents. Rise in obesity, standard of living, and income inequality (as measured by the Gini index) increased concomitantly.