In this study of the effects of maternal diabetes on the growth of offspring, 52 diabetic mothers were enrolled prenatally in the Providence cohort of the Perinatal Collaborative Study during 1960-1963. Among the offspring, there were 12 perinatal deaths. We were able to enroll 34 survivors and 34 matched controls in an adolescent follow-up study. Of the 34 diabetic mothers, six were insulin dependent, six had chemical diabetes, 13 had gestational diabetes, and nine were suspects. As part of the Collaborative Project, sequential follow-up evaluations had been done at four months, one year, four years, and seven years. A positive relationship was found between maternal pre-pregnant weight, weight gain during pregnancy, and the neonatal weight/height index in both diabetic and control subjects. At 7 years of age, eight of 19 offspring of diabetic mothers who were large for gestation at birth were obese, whereas only one of 14 infants who were appropriate for gestation at birth were obese (P < 0.05). Adolescent obesity (weight/height index greater than or equal to 1.2) was more likely in infants who had been LGA at birth. These data suggest that macrosomia in infants of diabetic mothers may be a predisposing factor for later obesity.