A double-blind controlled trial was carried out in patients with chronic complicated urinary tract infections in order to evaluate objectively the therapeutic effectiveness and side effects of piperacillin comparing with those of carbenicillin as a control. Consequently, the following results were obtained. 1. Of the 316 patients originally entered in the study, 244 patients were found to satisfy completely the clinical conditions excluding 61 cases of exception and 11 cases of drop-out. The background factors were examined and the clinical effectiveness were evaluated in these selected patients. It was found that the number of patients administered piperacillin was 127, and carbenicillin 117. 2. There was no significant difference in various background factors of patients between both groups, therefore it was considered that both groups were almost similar ones to be comparable each other. 3. The therapeutic dosis was 2 g/day in the piperacillin group and 4 g/day in the carbenicillin group. And, the clinical effects were evaluated following 5-day administration of each drug. The efficacies of both piperacillin and carbenicillin groups in the overall clinical evaluation were 44.1 % and 27.4%, respectively. This efficacy in the piperacillin group was significantly high. Though there was no difference in effectiveness on pyuria between both groups, a more significant effect was observed in the piperacillin group in effectiveness on bacteriuria in respect to the negative percentage value + reduction rate as well as the negative percentage value. 4. In overall assessmentc lassified by various factors of patients, the piperacillin group showed a significantly high efficacy in respect to the overall clinical effectiveness and the bacteriological negative percentage value + reduction rate as well as the negative percentage value in catheterized patients. 5. The side effects were examined in 129 patients in the piperacillin and 118 in the carbenicillin group. The subjective side effects were observed in only 3 cases (2.3%) in the piperacillin group and also 3 cases (2.5%) in the carbenicillin group, no difference being seen between both groups. No patient showed abnormal clinical findings in hematological and renal function tests. Abnormal value of GOT was observed in 2.0%, GPT in 1.4%, alkaline phosphatase in 0.7%, no significant difference being observed between both groups. Piperacillin was considered to be a safe drug with less side effects as well as carbenicillin. 6. In conclusion, the authors' experiences suggest that piperacillin is a useful drug for chronic complicated urinary tract infections.