A randomized comparison of aztreonam (2 g intravenously every eight hours) versus gentamicin (1.5 mg/kg intravenously every eight hours), each with clindamycin (600 mg intravenously every six hours), was performed in 119 patients with endometritis after cesarean section. Patients in both groups had similar risk factors. Genital cultures revealed an average of 3.0 isolates per specimen. Eighty-five aerobic gram-negative rods were isolated from 57 (48%) patients. All were susceptible to both aztreonam and gentamicin. Of 133 anaerobic isolates, 131 (98%) were susceptible to clindamycin. The failures in the aztreonam group were associated with a wound abscess and with an enterococcal bacteremia. Of the six failures in the gentamicin group, two were associated with persistent isolation of enteric bacilli. In the other four failures, no explanation was evident. Side effects occurred in four patients, (three diarrhea, one allergic reaction). All were self-limited and appeared to be due to clindamycin. No patient showed nephrotoxicity. When used in combination with clindamycin, aztreonam gave clinical results similar to gentamicin.