To determine the incidence and risk factors associated with an abnormal postpartum glucose tolerance in women with gestational diabetes, 103 patients with gestational diabetes had a 2-hour, 75 gm oral glucose tolerance test 6 +/- 2 weeks (mean +/- SD) after delivery. Twenty-two percent (23/103) of results were abnormal: Three showed frank diabetes, four showed impaired glucose tolerance, and 16 were, nondiagnostic. There was a significant difference in gravidity, pregravid weight and body mass index, delivery weight, gestational age at diagnosis, fasting and 2- and 3-hour glucose level at the time of the oral glucose tolerance test during pregnancy, need for insulin therapy during gestation, and neonatal weight greater than 4000 gm in the abnormal group as compared with the normal group. Elevated fasting glucose level (p = 0.0001) and earlier gestational age at time of diagnosis of gestational diabetes (p = 0.013) were found to be most predictive of an abnormal postpartum glucose tolerance test result. These results support the importance of postpartum oral glucose tolerance testing in women with gestational diabetes.