Four hundred and forty-five gestational diabetic women who were delivered over a 5-year period were evaluated. One hundred and fifteen were treated with insulin and diet, 184 with diet alone, and 146 with neither insulin nor dietary manipulation. The incidence of babies weighing more than 4000 gm was 7% in the insulin-treated group, 18.5% in the diet-treated group, and 17.8% in the untreated group (p less than 0.05). The incidence of operative delivery (midforceps, midcavity vacuum extraction, or primary cesarean section) was 16.3% in the insulin-treated group, 30.4% in the diet-treated group, and 28.5% in the untreated group (p less than 0.05). The incidence of birth trauma (shoulder dystocia, Erb's palsy, cephalhematoma, soft tissue injury) was 4.8% in the insulin-treated group, 13.4% in the diet-treated group, and 20.4% in the untreated group (p less than 0.05). These data suggest that prophylactic insulin treatment prevents not only macrosomia but also its sequel of traumatic delivery.