Seventy-two patient with gestational diabetes were randomly treated with insulin (20 units NPH and 10 units regular) and diabetic diet, diet alone, or neither. Of the 27 patients treated with insulin and diet, 2 (7%) had babies weighing more than 8 1/2 pounds. Of the 11 patients treated with diet alone, 4 (36.4%) had babies weighing more than 8 1/2 pounds. Of the 34 patients treated with neither diet nor insulin, 17 (50%) had babies weighing more than 8 1/2 pounds. These data support the hypothesis that treatment of the gestational diabetic with insulin will reduce the incidence of fetal macrosomia.