Longitudinal correlations were obtained between amniotic fluid insulin concentration at 32 to 38 weeks' gestation and anthropometric characteristics at the age of 6 years in 56 children of diabetic mothers. The prospective studies indicated that at the age of 6 years, as at birth, the greatest increase in weight in relation to height (relative obesity) was seen in children who experienced the greatest exposures to insulin in the uterus (as judged by amniotic fluid insulin concentration). Significant correlations between amniotic fluid insulin and relative obesity at the age of 6 years were found after adjustment for maternal obesity and macrosomia at birth. The highest amniotic fluid insulin values are clustered in the subgroup of 14 children who were obviously obese by the age of 6 years. These findings are consistent with the hypothesis that there is an association between anthropometric development and intrauterine metabolism, and suggest that premature and excessive exposure to insulin during gestation may predispose to obesity in childhood. The amniotic fluid insulin concentration may predict this eventuality.