Five-hundred eighty eight bacterial strains isolated from 435 patients diagnosed as having urinary tract infections (UTIs) in 10 institutions in Japan were supplied between August 2000 and July 2001. Then, the clinical background of patients were investigated such as sex, age, and type of infections, 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. About the relationship between age and sex of patients and type of infections, the number of male patients aged less than 50 years was few, and complicated UTIs without indwelling catheter was the most frequent. In females, the number of patients aged less than 20 years was few. The majority of female patients aged 40 years and over had complicated UTIs while uncomplicated UTIs was most frequent in the patients being twenties. As for type of infections and kind of bacteria, Escherichia coli decreased when the infections became complicated, and pseudomonas aeruginosa increased when the infection became complicated. Enterococcus faecalis was isolated more frequently in complicated UTIs than in uncomplicated UTIs. Considering this result by age of patients, isolated frequency of E. coli was gradually decreased with aging in patients aged 20 years and over with uncomplicated UTIs or complicated UTIs without indwelling catheter. The isolated frequencies of Klebsiella spp., P. aeruginosa, and E. faecalis were high in the patients with complicated UTIs without indwelling catheter. In the patients aged 70 years and over with complicated UTIs with indwelling catheter, P. aeruginosa and E. faecalis were frequently isolated. As for type of causative organisms in UTIs before and after the administration of antibiotics, the isolation of bacteria was remarkably decreased after administration in the patients with uncomplicated UTIs and complicated UTIs without indwelling catheter. E. coli decreased after administration of antibiotics, and P. aeruginosa increased after administration in the patients with all types of infections. As for type of causative organisms in UTIs and surgical procedures, E. coli was frequently isolated in the patients without surgery in all types of infections, while P. aeruginosa was frequently isolated in the patients who underwent surgery. In uncomplicated UTIs, Proteus spp. and E. faecalis were frequently isolated in the patients with surgery. In complicated UTIs without indwelling catheter, Klebsiella spp. was frequently isolated in the patients without surgery and E. faecalis was frequently isolated in the patients with surgery. In complicated UTIs with indwelling catheter, most of organisms except P. aeruginosa and S. aureus were frequently isolated in the patients without surgery.