BACKGROUND: We studied the antibiotic sensitivities to the causative microorganisms of acute simple urinary tract infection for recent 3 years. METHODS: We analyzed 112 microorganisms and their antibiotic sensitivities of the 104 patients who were admitted to or visited the Department of Urology, Catholic University St. Marys Hospital and had more than 10cfu/mL on urine culture from June 1996 to January 1999 retrospectively. RESULTS: The chance of gram negative and positive as causative microorganisms was 72.3% and 27.7% respectively. The most common pathogenic microorganisms were Escherichia coli (67.0%) followed by a-hemolytic streptococci, Entercoccus. In gram negative acute UTI, imipenem, sulperanzone showed relatively higher sensitivity, while cotrimoxazole, ampicillin showed relatively lower sensitivity. In gram positive, vancomycin, penicillin showed relatively higher sensitivity, while ampicillin, imipenem showed relatively lower sensitivity. CONCLUSIONS: We consider that gram negative microorganisms, especially E. coli, is the main cause of acute simple UTI. But, we should be concerned about the increase of gram positive organisms and other gram negative organisms besides E. coli. Regarding to the choice of adequate drug in the treatment of UTI, it is necessary to consider the change of pathologic microorganisms.