During a 4-month period 265 women delivered by cesarean section were studied to determine what effect membrane rupture has on the incidence and severity of postoperative infection. There was a definite correlation between the duration of ruptured membranes and the incidence as well as severity of postoperative infections. Only 29% of women with intact membranes subsequently developed endometritis with pelvic cellulitis, in contrast to 85% of those whose membranes were ruptured for >6 hours. Wound and pelvic abscesses were encountered in less than 1% of women delivered with intact membranes, yet these complications developed in over 30% of women with membranes ruptured for >6 hours. The incidence of septicemia was four times greater in those women whose membranes were ruptured for >6 hours. Women with endometritis were treated with one of two empirical antimicrobial regimens chosen randomly. Intravenous penicillin and tetracycline was found to be as effective as, and perhaps slightly more effective than, the combination of intravenous penicillin and intramuscular tobramycin. © 1978 The American College of Obstetricians and Gynecologists.