Clinical background was investigated on patients with urinary tract infections (UTIs) from whom 603 bacterial strains were isolated in 9 hospitals during the period from June, 1997 through May, 1998. 1. Distribution of age and sex of patients and type of infections Among males, those with ages less than 50 years were only 12.5%, and those in the 70's were most frequent (33.3%). Among females, those with ages less than 20 years were only 3.4%, and those in the 60's were the most frequent (22.9%). With regard to type of infections, more than a half of infections among males with ages 50 years or older were of complicated types, but most of infections among females were of uncomplicated types, especially those of ages less than 50 years. 2. Ages of patients and types of pathogens Escherichia coli was the most frequently isolated in uncomplicated and complicated UTIs without indwelling catheters. In complicated UTIs with indwelling catheters, Pseudomonas aeruginosa and Enterococcus faecalis had mainly detected, they had no relationship with ages. 3. Effect of antibiotic use on isolation frequencies of pathogens Use of antibiotics decreased pathogens isolated from patients with uncomplicated UTIs drastically (183 isolates before antibiotics compared to 13 after). Even isolated pathogens from patients with complicated UTIs decreased drastically with the use of antibiotics when indwelling catheters were not in use (175 isolates before antibiotics compared to 53 after), but when indwelling catheters were in use, antibiotics slightly decreased (109 isolates before antibiotics compared to 70 after). 4. Surgical procedures and types of causative organisms for UTIs E. faecalis were more isolated when a surgical procedures were used, and E. coli were more isolated when, they were not used in uncomplicated and complicated UTIs without indwelling catheters. In complicated UTIs with indwelling catheters, E. coli and E. faecalis were more isolated when a surgical procedures were used, and P. aeruginosa and Staphylococcus aureus were more isolated when they were not used.