We carried out a randomized multi-center study comparing cefpirome (CPR) 0.5 g b.i.d. (1 g group), 1.0 g b.i.d. (2 g group) and ceftazidime (CAZ) 1.0 g b.i.d. (CAZ group) in the treatment of complicated urinary tract infections. Patients who were over 16 years old and had underlying urinary tract disease, with bacteriuria of more than 10(4) cells ml or more and pyuria of more than 5 WBCs/hpf (x 400) or more were randomly allocated to receive either 0.5 g of CPR, 1.0 g of CPR or 1.0 g of CAZ twice a day for 5 days by intravenous drip infusion. The overall clinical efficacy of the treatment was evaluated by the criteria of the Japanese UTI Committee as excellent, moderate or poor, on the basis of the changes in pyuria and bacteriuria. A total of 530 patients were treated. Of these, 141 patients in the 1 g group, 136 in the 2 g group, and 140 in the CAZ group were evaluable for clinical efficacy. No significant differences in background characteristics were observed among the treatment groups. The overall clinical efficacy rate of the 1 g group, the 2 g group and the CAZ group was 80.1%, 76.5% and 71.4%, respectively. The differences were not statistically significant. The overall bacteriological eradication rate of the 1 g group, the 2 g group and the CAZ group was 81.0%, 88.1% and 83.8%. The differences were not statistically significant either. Against the enterococcus group, however, eradication rates were higher significantly in the 1 g and 2 g groups than in the CAZ group. The incidence of adverse reactions was 2.2% in the 1 g group, 0.6% in the 2 g group and 2.9% in the CAZ group. Abnormal laboratory data after medication were observed in 10.8% of the 1 g group, 12.1% of the 2 g group and 10.2% of the CAZ group, the difference not being statistically significant. There were no serious untoward reactions to medication. From the results obtained in this study, we consider that CPR is at least as useful as CAZ in the treatment of complicated urinary tract infections.