A prospective, randomized clinical trial was conducted in 103 patients undergoing cesarian section to assess the efficacy of prophylactic, intravenously administered Metronidazole on the infectious morbidity. A group of 53 patients with perioperative Metronidazol-prophylaxis was compared to a similar controll-group without prophylaxis. Bacteriologic swabs were taken from the cervix pre- and postoperatively, using anaerobic transport media. Prophylactic Metronidazole reduced postoperative fever of more than 38 degrees C on two subsequent days from 60% in the controll-group to 30,2% in the Metronidazole-group (p less than 0,01) wound infections were reduced from 18% without to 5,7% with prophylaxis (p less than 0,05) and Endometritis from 30% without to 13,2% with prophylaxis (p less than 0,05). An additional antibiotic therapy was necessary in 44% of the cases in the controllgroup, compared to 24,5% of the cases in the Metronidazolegroup (p less than 0,05). The mean duration of hospitalisation was reduced from 12,1 +/- 3,2 days in the controll-group to 11,2 +/- 2,1 in the Metronidazole-group (p less than 0,01). Anaerobic bacteria were isolated from the servical swabs in 60% preoperatively, with a still increasing incidence to 72% postoperatively, compared to 7% in the Metronidazole-group. Our results suggest, that prophylactic, intravenously administered Metronidazol reduces the infectious morbidity following cesarian section due to the reduction of the anaerobic flora at the female genital-tract.