The prevalence of obesity was determined in 789 9 to 15-year-old Philadelphia subjects who had either a high (+1 SD) or a low (-1, SD) relative weight at 1 year of age. During the adolescent years, obesity was assessed by the triceps skin-fold thickness and by the relative weight, using national reference standards from the US Health Examination Survey. The prevalence of obesity for the high relative weight group at 1 year of age was three to four times higher than in the low relative weight group at 1 year of age. Compared with all urban youth of this age range, regardless of their status at 1 year of age, it is estimated that the risk ratio associated with a high relative weight at 1 year of age is approximately 1.6.