Infants of diabetic mothers are considered to be at increased risk of fetal distress. We suggest that umbilical vasculitis is strongly associated with abnormal fetal rate patterns, which indicate fetal tissue hypoxia even in the absence of clinical evidence of intrapartum infection. In the present series, 132 term deliveries with Class A diabetes mellitus were compared with 159 uncomplicated term deliveries. Umbilical vasculitis and gestational age greater than 281 days were both highly correlated with abnormal fetal heart rate patterns in diabetic pregnancies (p less than 0.001). These effects were shown to be independent by multiple logistic regression. There was no difference in mean birthweight between infants of diabetic mothers and those of uncomplicated pregnancies, but infants of diabetic mothers had heavier placentas (p less than 0.01). These data further support our hypothesis that umbilical vasculitis may affect umbilical hemodynamics and predispose to development of fetal heart rate patterns.