Seven hundred and nineteen bacterial strains isolated from 565 patients diagnosed as having urinary tract infections (UTIs) in 14 institutions in Japan were collected between August 2002 and July 2003. The frequency of bacteria isolation divided with patient clinical background was compared. The clinical background investigated included sex, age, type of infections, timing of antibiotics administration, and presence or absence of surgery affecting a decrease in defense against infection. The bacterial strains were divided with 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. Number of patients aged 20-39 years was greater in female than male. In all of ages except 0-9 and 70-79 years, the ratio of the uncomplicated UTIs was high, accounting for 44.4-91.7% of all types of infections. In the present time, the bacteria most frequently isolated were Escherichia coli, Pseudomonas aeruginosa and Enterococcus faecalis also were relatively frequently isolated. E. coli was most frequently isolated from the uncomplicated UTIs, and P. aeruginosa and E. faecalis were frequently isolated from the complicated UTIs with indwelling catheter. 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 40% or higher. In the complicated UTIs without indwelling catheter, the isolation frequency of E. coli decreased with aging of the patients but still was the highest in all age groups. 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, P. aeruginosa increased after the administration in any types of 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 and E. faecalis were more frequently isolated in the patients with surgery in any UTIs.