OBJECTIVE To determine 1) whether macrosomia in the fetus of the diabetic mother is related to fetal hyperinsulinemia and 2) whether hyperinsulinemia and macrosomia are related to maternal metabolic control. RESEARCH DESIGN AND METHODS Normal pregnant women (n = 95) were compared with insulin-treated pregnant women (n = 155), who were subdivided according to White9s class, hypertension, and mode of delivery. All women were treated to achieve optimal metabolic control. HbA1c was determined at each visit. At delivery, umbilical plasma was analyzed for glucose, insulin antibodies, total insulin, free insulin, C-peptide, proinsulin components, and total and individual amino acids. RESULTS Macrosomia, defined as <2 standard deviation units (97.75%), was found in 10–27% of the diabetic groups. It was not related to maternal mass or size, but was significantly correlated with umbilical total insulin, free insulin, and C-peptide. Proinsulin components were not different among groups. Amino acids also were not different. Glycosylated hemoglobin was a weak predictor of birth weight and fetal hyperinsulinism. CONCLUSIONS Macrosomia in the fetus of the diabetic mother remains inadequately explained. In a large population of pregnant women with strict metabolic control, macrosomia was mainly independent of glycosylated hemoglobin. Nevertheless, fetal hyperinsulinism remains the driving force for excessive fetal growth. The stimulus for fetal insulin excess in humans remains to be defined.