The objective was to investigate the hypothesis that anthropometric and body composition differences exist between macrosomic infants of diabetic and nondiabetic mothers. Sixteen infants of mothers with diabetes, along with 58 control infants, were studied within 24 hours of delivery. The following measurements were obtained: birthweight, birth length and extremity length; circumferences of the head, chest, shoulders, and extremities; and triceps, subscapular, flank, and thigh skinfolds. Estimation of fat mass and calculation of percent body fat was performed according to the Dauncey method. Macrosomic infants of diabetic mothers were characterized by larger shoulder and extremity circumferences, a decreased head-to-shoulder ratio, significantly higher body fat, and thicker upper extremity skinfolds compared with nondiabetic control infants of similar birthweight and birth length. Differences in body composition and weight distribution may explain the propensity for shoulder dystocia in the diabetic population.