An increase in fetal and maternal complications has been documented in cases of gestational diabetes, but the glucose levels that predict an increased risk have not been clearly defined. We evaluated the frequency of several neonatal complications (macrosomia, congenital anomalies, perinatal mortality, and prematurity) and maternal complications (toxemia, cesarean section, or both) in relation to glucose tolerance in 249 women in the third trimester of pregnancy. None of the women had previous evidence of diabetes, and all had normal results on an oral glucose-tolerance test, according to accepted criteria. On the basis of their two-hour plasma glucose levels, women were divided into three groups: A (glucose less than 100 mg per deciliter), B (glucose 100 to 119 mg per deciliter), and C (glucose 120 to 164 mg per deciliter). The higher two-hour plasma glucose levels were associated with a significant increase in the incidence of macrosomia (9.9, 15.5, and 27.5 percent in Groups A, B, and C, respectively), congenital abnormalities (0.7, 3.5, and 5.0 percent), and toxemia, cesarean section, or both (19.9, 25.9, and 40.0 percent). A significant correlation between the infant's weight and the mother's two-hour plasma glucose level was also observed. These data indicate that even limited degrees of maternal hyperglycemia, which are currently considered to be within the normal range, may affect the outcome of pregnancy.