Piperacillin, a new semisynthetic penicillin, has broad spectrum activity against most clinically important aerobic and anaerobic bacteria. In the present study, piperacillin was compared with a combination of clindamycin and gentamicin for the treatment of 83 women with pelvic infection (42 with endometritis, 29 with posthysterectomy cuff infections, 11 with acute salpingitis, and one with a wound infection). There were 179 bacterial isolates, 98 (53%) aerobic and 81 (45%) anaerobic. Of the 42 patients treated with piperacillin, there were three (7.1%) clinical failures compared with one (2.4%) in the 41 patients treated with clindamycin-gentamicin. A single drug, piperacillin, was shown to be as safe and effective as the combined clindamycin plus gentamicin therapy for pelvic infections.