To determine the prevalence of abnormal carbohydrate metabolism in the early postpartum period in women with gestational diabetes mellitus, we performed 2-hour oral glucose tolerance tests between 5 and 8 weeks post partum in 246 women with recent gestational diabetes mellitus. Patients were stratified into three study groups based on their fasting serum glucose level during pregnancy: (1) group A1: all fasting serum glucose levels during pregnancy less than 105 mg/dl without insulin therapy; (2) group A2: any fasting serum glucose levels greater than 105 and less than 140 mg/dl before insulin therapy; (3) group B1: any pregnancy with fasting serum glucose levels greater than 140 mg/dl. Overall, 48 (19%) of the patients had an abnormal oral glucose tolerance test in the early postpartum period; 25 (10%) had impaired glucose tolerance and 23 (9%) had diabetes mellitus. The prevalence of postpartum diabetes mellitus (2% in group A1, 9% in group A2 and 44% in group B1) increased in parallel with the degree of maternal metabolic decompensation during pregnancy (p less than 0.05 for A1 versus A2; p less than 0.001 for A2 versus B1). The prevalence of impaired glucose tolerance was likewise greater in the B1 group (26%) than in either the A1 or the A2 group (p less than 0.05). Gestational age less than 24 weeks at diagnosis of gestational diabetes mellitus was an additional risk factor for postpartum glucose intolerance. Our findings support the use of an oral glucose tolerance test in the early puerperium, especially for patients with elevated fasting serum glucose levels during pregnancy.