A total of 192 men and 139 women aged 15 to 89 years with diagnosed intra-abdominal infection were randomised in a 2:1 ratio to treatment with either intravenous piperacillin/tazobactam (3 g/375 mg every six hours) or clindamycin (600 mg every six hours) plus gentamicin (2.5 mg to 5.0 mg/kg every eight to 12 hours) in a multicentre trial. Of 147 evaluable patients with microbiologically confirmed infections, 104 were treated with piperacillin/tazobactam and 43 with clindamycin plus gentamicin. The diagnoses of perforated appendicitis (n = 79), other peritonitis (n = 32), cholecystitis/cholangitis (n = 18), intraabdominal abscess (n = 14), and diverticulitis (n = 3), were distributed proportionately between the two therapeutic groups. Ninety one of 104 patients (88%) in the piperacillin/tazobactam group and 33 of 43 patients (77%) in the clindamycin plus gentamicin group were considered cured or improved (p = 0.13). In the piperacillin/tazobactam group, 80 of 88 (91%) Bacteroides fragilis group organisms and 68 of 74 (92%) E coli isolates were eradicated; in the clindamycin plus gentamicin group, 21 of 25 (84%) Bacteroides fragilis group isolates and 23 of 30 (76%) E coli isolates were eradicated. Eleven evaluable patients in the piperacillin/tazobactam group had beta-lactamase-producing organisms that were resistant to piperacillin but susceptible to piperacillin/tazobactam; in 10 of these patients (91%) bacteria were eradicated. We conclude that piperacillin/tazobactam is an effective antimicrobial drug for monotherapy of intra-abdominal infections, with efficacy similar to or better than standard aminoglycoside/anti-anaerobe combinations.