For the purpose of carrying out an objective evaluation of the clinical efficacy and safety of cefotetan (CTT), a new cephamycin-type injectable antibiotic, in the treatment of complicated urinary tract infections, cefmetazole (CMZ) was employed as the control drug in a comparative study performed by the double-blind method. CTT was administered in a dose of 0.5 g or 1 g, while CMZ was used in a dose of 1 g; both of these drugs were administered by intravenous drip infusion twice a day for 5 days. The total number of treated cases was 393; 105 patients in the CTT 1 g group, 97 patients in the CTT 2 g group, and 96 patients in the CMZ 2 g group. The judgment of the clinical effects of these 3 therapeutic regimens was carried out in accordance with the criteria for evaluation of clinical efficacy of antimicrobial agents on UTI (2nd edition). The results are as follows. With regard to the overall efficacy of the drug treatments, the calculated efficacy rates were 54.3% in the CTT 1 g group, 63.9% in the CTT 2 g group and 52.1% in the CMZ 2 g group. The differences between these efficacy rates were not statistically significant. Regarding the efficacy of the treatments in terms of the bacteriuria, the bacterial cultures were seen to become negative in 40% of the cases in the CTT 1 g group, 54.6% of the CTT 2 g group and 42.7% of the CMZ 2 g group. Again, there were no statistically significant differences between the groups. Concerning the efficacy of the drugs against pyuria, by combining the cases found to be normalized and those seen to show improvement, the improvement rates were calculated to be 41.9% for the CTT 1 g group, 37.1% for the CTT 2 g group, and 37.5% for the CMZ 2 g group. These pyuria improvement rates show no statistically significant differences, respectively. The incidences of occurrence of side effects and laboratory test abnormalities thought to be caused by the drug therapy were as follows; 1.5% and 5.3% in the CTT 1 g group, 1.5% and 3.1% in the CTT 2 g group and 0% and 6.9% in the CMZ 2 g group. There were no significant differences between the groups. Based on the above clinical results, it can be concluded that, in the treatment of complicated urinary tract infections, we can expect CTT to provide the same therapeutic efficacy as CMZ when its dosage is the same or even half thereof. In addition, the safety of CTT is equal to that of CMZ. Therefore, CTT is concluded to be a useful drug.