To determine the incidence of fetomaternal hemorrhage in patients undergoing cesarean section, Kleihauer-Betke tests were performed in the immediate postoperative period on 199 parturients. Some degree of hemorrhage was detected in 18.5% of patients, with 2.5% demonstrating greater than 30 ml of fetal blood. Comparison of groups on the basis of indication for cesarean delivery revealed no difference in rates of fetal hemorrhage. Because patients with greater than 30 ml of fetal blood would not be adequately protected from Rh sensitization by the standard 300 micrograms dose of Rh immune globulin, we recommend screening all Rh-negative patients undergoing cesarean section for the presence of significant fetomaternal hemorrhage.