Five-hundred forty four bacterial strains isolated from 412 patients diagnosed as having urinary tract infections (UTIs) in 9 institutions in Japan were supplied between the period of August 1999 to July 2000. Then, the clinical background of patients were investigated such as sex, age and type of infections, infections and kind of bacteria, frequency of isolation of bacteria by age and infections, bacteria and infections by timing of administration of antibiotics, 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. Complicated UTIs without indwelling catheter was the most frequent among female patients aged between 40 to 59 years, in other age groups, uncomplicated UTIs was most frequent. As for type of infections and kind of bacteria, Escherichia coli decreased when the infections became complicated, and Pseudomonas aeruginosa and Enterococcus faecalis increased when the infection became complicated. Considering this result by age of patients, isolation frequency of E. coli was gradually decreased with aging in patients aged more than 20 years with uncomplicated UTIs or complicated UTIs without indwelling catheter. The isolation frequencies of E. faecalis and Staphylococcus aureus were gradually increased with aging in complicated UTIs without indwelling catheter. In patients with complicated UTIs with indwelling catheter, there was no difference between age group, and 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 patients with uncomplicated UTIs and complicated UTIs without indwelling catheter. E. coli decreased after administration of antibiotics, and P. aeruginosa and E. faecalis increased after administration in patients with all infections. As for type of causative organisms in UTIs and surgical procedures, E. coli were more frequently isolated in patients with uncomplicated UTIs when surgical procedures were experienced. Also, Klebsiella spp. and E. faecalis were more frequently isolated in patients with surgical procedures. However, in complicated UTIs, type of causative organisms had no relationship with surgical procedures.