Piperacillin/tazobactam, at a dosage of 4 g/500 mg every 8 h, was administered intravenously to 217 patients with complicated urinary tract infections. The most common diagnosis was pyelonephritis. The most common pathogen was Escherichia coli (47%) followed by Pseudomonas aeruginosa (13%), and enterococci (8%). Among clinically evaluable patients, 86% (115/134) were cured or improved at the study endpoint and 14% (19/134) were clinical failures or relapsed. Among bacteriologically evaluable patients, 85% (95/112) had a favorable clinical response at endpoint. The bacteriological response rate was 73% (82/112) at endpoint. Overall, 82% of all pathogens were eradicated. Therapy was associated with a low incidence of side effects, and adverse experience were mild and of short duration.