Objective To assess the efficacy of single-dose antibiotic prophylaxis against postpartum endomyometritis in high-risk cesarean section patients. Design Patients were administered one of three single-dose antibiotic regimens following umbilical cord clamping after cesarean section delivery. Setting Prospective randomized trial at a university-based hospital. Patients The study evaluated 293 consenting women undergoing cesarean section who had either experienced labor for a duration of 6 hr or rupture of amniotic membranes. Main outcome measures Development of postpartum endomyometritis. Results The incidence of postpartum endomyometritis was 7/95 (7.4%) following the ampicillin/sulbactam regimen, 14/98 (14.3%) after the cefazolin regimen, and 11/99 (11.1%) after the cefotetan regimen. There was no significant difference in postpartum infection among the three study arms. In addition, the incidence of endomyometritis in the three single-dose study arms was not higher than previously noted in studies where three doses of antibiotic were administered. Conclusion Single-dose antibiotic prophylaxis should replace the standard triple-dose therapy for uninfected women undergoing cesarean section who are at risk for postoperative endomyometritis. Ampicillin/sulbactam, cefazolin, and cefotetan are all reasonable antibiotic choices for single-dose therapy. Infect. Dis. Obstet. Gynecol. 6:220–223, 1998. © 1998 Wiley-Liss, Inc.