The current multicenter study was conducted at five sites using 86 patients to evaluate the safety and efficacy of piperacillin/tazobactam (4 grams per 500 milligrams every eight hours) compared with imipenem/cilastatin (1 gram every eight hours) in the treatment of patients who were hospitalized with a clinically or bacteriologically confirmed diagnosis of intra-abdominal infection. Forty-seven patients received piperacillin/tazobactam and 39 received imipenem/cilastatin. The favorable response among patients who were clinically evaluable with a valid response in the group treated with piperacillin/tazobactam was 87 percent. In the group treated with imipenem/cilastatin it was 77 percent. Bacteriologic eradication rate among bacteriologically evaluable patients with a valid response in the group treated with piperacillin/tazobactam was 100 percent. In the group treated with imipenem/cilastatin it was 89 percent. The eradication rate of pathogens isolated from patients who were evaluable by biologic factors in the group treated with piperacillin/tazobactam was 100 percent and in the group treated with imipenem/cilastatin treatment, 96 percent. In the group treated with piperacillin/tazobactam the incidence and type of adverse reactions were similar to those seen with piperacillin alone. It is concluded that piperacillin/tazobactam is safe and efficacious in the treatment of patients hospitalized with intraabdominal infections and that tazobactam extends the spectrum of piperacillin.