A total of 615 bacterial strains isolated from 491 patients diagnosed as having urinary tract infections (UTIs) in 13 institutions in Japan were supplied between August 2002 and July 2003. Then, the clinical background of patients were investigated such as sex, age, and type of infections and kind of bacteria, frequency of bacteria isolation by age and infections, bacteria and infections by timing of antibiotics administration, and bacteria and infections by surgical procedures. The bacterial strains were divided by the age and sex of the patients and the types of infections. In males, the number of patients aged less than 50 years was few and the complicated UTIs without indwelling catheter was observed most frequently. In the present time, the bacteria most frequently isolated were Escherichia coli. Pseudomonas aeruginosa and Enterococcus faecalis were also relatively frequently isolated. E. coli decreased and P. aeruginosa increased with the complication of the infections. E. faecalis tended to be isolated more frequently in the complicated UTIs than in the uncomplicated UTIs. With respect to the relation of these results to the age of the patients, in the uncomplicated UTIs, the isolation frequency of E. coli was the highest in all age groups, accounting for 70.8% in 20-49 years old. In the complicated UTIs without indwelling catheter, the isolation frequency of E. coli decreased with aging of the patients and was lower than that of E. faecalis in the patients aged 70 or more (18.3 vs 20.6%). In the complicated UTIs with indwelling catheter, the isolation frequency of E. coli was lower than in the uncomplicated UTIs in all age groups and P. aeruginosa and E. faecalis were more frequently isolated. In comparison of causative bacteria in UTIs between before and after the administration of antibiotics, E. coli decreased and P. aeruginosa increased after the administration in any UTIs. In comparison of causative bacteria in UTIs with or without surgery, E. coli was more frequently isolated in the patients without surgery, while P. aeruginosa was more in the patients with surgery in any UTIs. E. faecalis was more frequently isolated in the patients with surgery in uncomplicated and complicated UTIs without indwelling catheter and in the patients without surgery in complicated UTIs with indwelling catheter.