We compared maternal and neonatal outcomes in women who received prophylactic antibiotics prior to skin incision to those who received antibiotics at cord clamp. We performed a randomized clinical trial at two sites. Eligible women included those undergoing nonemergency cesarean at 36 weeks' gestation or greater. Subjects were randomized (permuted blocks) into one of two treatments: "preoperative antibioticso" (cefazolin 1 g given <30 minutes prior to skin incision) or "intraoperative antibioticso" (cefazolin 1 g at cord clamping). Patients who reported an allergy to penicillin received clindamycin 900 mg. The trial primary outcome was a composite of maternal infectious morbidities, defined as having any one of the following: (1) postoperative fever (defined as oral temperature >38C on two separate occasions more than 6 hours apart, after the initial 24-hour postoperative period); (2) wound infection (defined as purulent discharge from the incision); (3) endomyometritis (defined as fundal tenderness and fever malodorous lochia, fever); (4) urinary tract infection (defined as fever, positive urine culture). We enrolled a total of 434 subjects in this study, with 217 in each group. Overall, we found no difference in composite maternal infectious morbidity between those who received antibiotics preoperatively and those who received antibiotics at cord clamp (relative risk = 1.2, 95% confidence interval 0.7 to 1.5). Neonatal outcomes were also similar between the two intervention arms. The rate of suspected sepsis was similar between the two groups. There were no cases of antibiotic resistance in the neonates. Either preoperative antibiotic therapy or antibiotic administration after cord clamp is a reasonable clinical method for reducing the risk of postcesarean infectious morbidity.