A total of 60 children with secondary peritonitis were enrolled in an open, non-comparative multicenter study designed to evaluate the safety, tolerance and efficacy of parenteral piperacillin/tazobactam (80/10 mg/kg every 8 hours) in young children. The most common diagnosis was perforated appendicitis (90%) and the three most common pathogens, obtained from the peritoneal cavity, were Escherichia coli (52 isolates), Pseudomonas aeruginosa (16 isolates) and Bacteroides sp. (19 isolates). Patients were examined daily during therapy, 4-14 days and 4-6 weeks post-therapy. Of the 60 patients, 43 were evaluable. The majority of patients had polymicrobial infections (36 patients). All the aerobic isolates were susceptible to piperacillin/tazobactam while 19 were resistant to piperacillin alone. Four of 43 clinically evaluable patients were considered a clinical failure and 3 of 40 bacteriologically evaluable patients were considered to have an unfavorable microbiological response. There were 2 clinically adverse events considered related to the study drug and several possibly related, mild and transitory, abnormalities in eosinophil counts and liver function tests. Based on the safety and efficacy results from this study, the advantages of using a single agent for the treatment of mixed infections of the peritoneal cavity and its potential activity against resistant organisms, we believe that further comparative clinical trials in children with intra-abdominal infections are warranted.