Changes of bacterial isolates from urine specimens of outpatients with urinary tract infections (UTI) from 1977 through 1984 were studied. Organisms which were isolated from patients with bacteriuria of over 10(3) bacteria per ml of urine regardless of grade of pyuria were studied. The incidence of UTI, especially that of acute cystitis was decreased during the recent 8 years. E. coli was the most predominant isolate in acute UTI with an isolation frequency of over 70% every year, followed by S. epidermidis nearly every year. Citrobacter spp., Klebsiella spp., P. mirabilis and S. aureus were also isolated steadily every year. Bacterial isolates in acute UTI were generally composed of these six species of bacteria. Although many kinds of bacteria were isolated in chronic UTI, E. coli was the most frequent species with an isolation frequency of 17-37%, followed by E. faecalis except in 1981 and 1982. The tendency of gram-negative rods to increase in acute and chronic UTI was observed except in 1981 and 1982. A slight decrease in sensitivity of E. coli and P. mirabilis against ampicillin (ABPC) and sulbenicillin (SBPC) was observed in acute UTI. In chronic UTI, the tendency of the sensitivity of E. coli against ABPC, SBPC and nalidixic acid to recover and the tendency of the sensitivity of P. aeruginosa against dibekacin to decrease were observed. E. faecalis was estimated to be sensitive to gentamicin, cefazolin, minocycline and fosfomycin. However, unlike those species mentioned above, many strains of E. faecalis showed a positive disc sensitivity to these drugs.