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 > 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.