To study the effectiveness of anaerobic coverage in prevention of postpartum endometritis in women undergoing nonelective cesarean sections, we conducted a randomized prospective double-blind study of women undergoing cesarean sections and requiring antibiotic prophylaxis from April 1, 1989, through December 31, 1990. Ninety-four patients were enrolled in the study. Forty-five patients received ampicillin alone and 46 received ampicillin in conjunction with sulbactam. All patients were evaluated prior to surgery and in the postoperative period. Ninety-one patients completed the study and their records were analyzed. Patients were divided into two groups depending on the presence or absence of ruptured membranes. Seventy-five percent of patients had ruptured membranes. Failure of prophylaxis and subsequent endometritis was documented in 8.8% of patients who received ampicillin and sulbactam and 35.3% of patients who received ampicillin alone. This difference was statistically significant (p < 0.02). In conclusion, single-dose ampicillin and sulbactam provides better prophylaxis than single-dose ampicillin in women undergoing cesarean section with rupture of membranes.