The most common adverse outcome associated with vaginal delivery is endometritis. It plays a significant role in postpartum morbidity and mortality. There is considerable evidence to support the idea that a single dose of antibiotic after vaginal delivery might decrease the incidence of postpartum endometritis. In this study the evaluation of the efficacy of antibiotic prophylaxis was based upon comparison of a group of patients given a single dose of Amox-CA (Augmentin) with a group of patients without treatment. The study was performed in the Department of Obstetrics and Gynecology of the A. Béclère Public Hospital, Clamart, France (Paris-Sud University). The patients who were the subject of the study had delivered vaginally during the period of 1 year, and were free of any clinical diagnosis of chorioamnionitis or other extragenital infection, had a maternal temperature of less than 38 degrees C during labor and 1 h after delivery, and had no history of allergy to penicillins or cephalosporins. After application of exclusion criteria, 1373 patients were randomized and 1291 included 610 in Group I given Amox-CA and 681 in Group II without any antibiotic. A single dose of 1.2 g of Amox-CA was given by intravenous injection, 1 h after delivery, in Group I. Patients of Group II received no injection. Postpartum status was evaluated before the patient left hospital and 2 weeks later. The two groups were similar in terms of demographic and clinical parameters. Four patients developed endometritis in Group I (4/610, 0.66%). Sixteen patients in Group II developed endometritis (16/680, 2.38%) (P = 0.013; 95% confidence interval (CI), 0.36-3.08%).(ABSTRACT TRUNCATED AT 250 WORDS)