The frequencies of isolation and susceptibilities to antimicrobial agents were investigated on 751 bacterial strains isolated from patients with urinary tract infections in 11 hospitals during the period of June 1991 to May 1992. Of the above total bacterial isolates, Gram-positive bacteria accounted for 28.6% and a majority of them were Enterococcus faecalis. Gram-negative bacteria accounted for 71.4% and most of them were Escherichia coli. 1. Enterococcus faecalis Ampicillin (ABPC), imipenem (IPM) and vancomycin (VCM) showed the highest activities against E. faecalis isolated from patients with urinary tract infections. The MIC90s of them were 2 micrograms/ml. Piperacillin (PIPC) and chloramphenicol (CP) were also active with the MIC90s of 8 micrograms/ml. The others were not so active with the MIC90s of 32 micrograms/ml or above. 2. Staphylococcus aureus including MRSA Arbekacin (ABK) showed the highest activities against S. aureus isolated from patients with urinary tract infections. Its MIC90 was 0.5 microgram/ml. VCM was also active with its MIC90 of 1 microgram/ml. The others were not so active with the MIC90s of 32 micrograms/ml or above. 3. Citrobacter freundii Ciprofloxacin (CPFX) showed the highest activities against C. freundii isolated from patients with urinary tract infections. Its MIC90 was 0.5 microgram/ml. IPM, gentamicin (GM), ABK and norfloxacin (NFLX) were also active with the MIC90s of 1 microgram/ml. Penicillins and cephems were not so active. 4. Enterobacter cloacae IPM showed the highest activities against E. cloacae isolated from patients with urinary tract infections. Its MIC90 was 0.5 microgram/ml. CPFX was also active with the MIC90 of 2 micrograms/ml. Aminoglycosides were active comparatively. The MIC90s of them were 4 micrograms/ml. Penicillins and cephems generally showed lower activities. 5. Escherichia coli IPM and ciprofloxacin (CPFX) showed the highest activities against E. coli isolated from patients with urinary tract infections. The MIC90s of them were 0.125 micrograms/ml or below. Flomoxef (FMOX), cefmenoxime (CMX), cefuzonam (CZON), latamoxef (LMOX), norfloxacin (NFLX) and ofloxacin (OFLX) were also active with the MIC90s of 0.25 microgram/ml. Penicillins except mecillinam (MPC) were not so active showing the MIC90s of 32 micrograms/ml or above. 6. Klebsiella pneumoniae IPM showed the highest activities against K. pneumoniae isolated from patients with urinary tract infections. Its MIC90 was 0.25 microgram/ml. Gentamicin (GM) and arbekacin (ABK) were also active with the MIC90s of 0.5 microgram/ml, respectively. But minocycline (MINO) and penicillins were not so active showing the MIC90s of 32 micrograms/ml or above. 7. Proteus mirabilis Most of the agents were active against P. mirabilis.(ABSTRACT TRUNCATED AT 400 WORDS)