The maternal antepartum, intrapartum, and neonatal characteristics of 158 patients with gestational diabetes mellitus (GDM) attending a large teaching hospital between 1979 and 1983 were described and compared with a matched nondiabetic control group. The primary cesarean section rate in patients with GDM (18%) was significantly greater than in the control group (11%, P < 0.04). Neonatal macrosomia, as reflected in mean birthweight (P < 0.04), the number of neonates weighing > 4 kg (P < 0.05) and large-for-gestationalage infants (P < 0.05), and the birthweight adjusted for gestational age (K-score, P < 0.01) was significantly increased in the diabetic group. The characteristics of patients with GDM treated with diet alone and diet and insulin together were examined. The insulin-therapy group was characterized by more patients older than 25 yr (P < 0.01) and a higher mean birthweight (3743 ± 677 g) (P < 0.02) than the dietalone group. This may reflect an increased magnitude of glucose intolerance in the insulin-treated group. Obese patients with GDM delivered heavier neonates than the nonobese patients with GDM (P < 0.01). Although there was no difference between the groups, perinatal mortality was present in this study. These data indicate that the major perinatal morbidity in GDM included increased cesarean section for fetal macrosomia. Early diagnosis with strict diagnostic criteria and rigid antenatal surveillance may result in further improvements in outcome.