The clinical efficacy and safety of cefotaxime and cefozolin in patients with complicated urinary ract infections with urinary underlying diseases were evaluated in a double blind trial carried out at the Department of Urology, School of Medicine, Kanazawa University and 14 affiliated hospitals.<BR>Patients of either sex and over 16 years of age were admitted into the study. Their pretreatment urinary leukocyte counts and urinary bacterial counts were at least 5 cells/hpf and 10⁴ cells/ml, respectively. The patients were randomly allotted into the cefotaxime group or cefazolin group. Patients were administered the assigned drug twice daily, in the morning and evening by i. v. injection for 5 consecutive days. Cefotaxime was administered in a daily dose of 2g and cefazolin 4g. Clinical efficacy was evaluated based on the effects on bacteriuria and pyuria according to the criteria set by the UTI committee in Japan.<BR>Two-hundred and sixty-four of 301 patients administered the assigned drug were enrolled into the evaluation of clinical efficacy. One-hundred and thiry-one received cefotaxime and 133 cefazolin. There was no significant difference in background factors such as age, sex, weight, site of infection, the number of catheterized patients or urinary leukocyte count between the two treatment groups. Both groups were therefore regarded to be homogeneous. Based on the improvement in bacteriuria and pyuria, the efficacy of cefotaxime was assessed to be excellent in 35 cases, moderate in 52 cases and poor in 44 cases for an effectiveness rate of 66%, while that of cefazolin was assessed to be excellent in 21 cases, moderate in 35 cases and poor in 77 cases for an effectiveness rate of 42%. Cefotaxime was thus significantly more effective than cefazolin.<BR>Bacteriologically, among all clinical isolates, cefotaxime eradicated significantly more organisms than did cefazolin. However, the bacteriological efficacies of cefotaxime and cefazolin were comparable against Eschericliia coli, Klebsiella, and indole-negative Proteus, organisms which are particularly susceptible to cefazolin.<BR>Adverse reactions occurred in 2 patients receiving cefazolin: One developed diarrhea which subsided after 4 days; the other developed an abnormal sensation of the mouth, facial flush and sneeze which lasted for several minutes after treatment. Laboratory abnormalities attributed to treatment consisted of 5 elevated transaminases and 1 elevated alkaline phosphatase in the cefotaxime group and 3 elevated transaminases and 2 elevated alkaline phosphatase in the cefazolin group. There were not significant differences. Their laboratory abnormalities normalized within one week after the termination of treatment.<BR>It was therefore concluded that 2g/day of cefotaxime were more effective than 4g/day of cefazolin in complicated urinary tract infections, with a comparable level of safety.