To evaluate the role of anaerobic bacteria in postpartum endomyometritis, intravenous metronidazole was used as the sole initial therapy in 25 women with clinical signs of infection. This agent has not been shown to have activity against aerobic pathogens in soft tissue pelvic infections. A clinical cure was obtained in 21 of 25 (84%) with oral metronidazole utilized after the patient had been afebrile for at least 24 hours. The mean fever index of the 21 successfully treated women was 35.9 degree hours with a SD of +/-30 hours. The four patients with treatment failure required alternate antibiotics and the mean fever index of these patients was 138.0 degree hours. Transcervical endometrial cultures were obtained prior to therapy in 24 women. Anaerobes were recovered in every instance with a total of 67 isolates, a mean of 2.8 per patient. The most common isolates were peptococcus and Bacteroides species. A culdocentesis was performed concurrently in 23 women. Anaerobes were recovered in 20 (87%) with a total of 55 isolates. The most frequent isolates were peptococcus and Bacteroides fragilis. In 23 women in whom paired transcervical endometrial and cul-de-sac cultures were obtained, at least one common organism was isolated in 18 (78%). Both the successful clinical response to metronidazole and the paired culture results suggest that anaerobic organisms have an important role in endomyometritis following vaginal delivery.