BACKGROUND: Quinolone antibiotics are now frequently used for the empirical therapy of acute pyelonephritis (APN). However, failure of this empirical therapy is often encountered in clinical practice, and the emergence of quinolone resistance should be concerned. This study was undertaken to investigate the prevalence of quinolone resistance in community-acquired APN and to analyze the clinical characteristics of quinolone-resistant APN. METHODS: Clinical data were retrospectively analyzed from 367 patients who were admitted to Hanyang University Hospital for APN from January 2002 through December 2004. According to the result of urine culture, the patients were divided into quinolone-susceptible and quinolone-resistant groups, and clinical characteristics were compared. RESULTS: Urine culture was positive in 241 out of 367 patients, and E. coli was the most common (90%) isolate. The prevalence of quinolone resistance based on the in vitro ciprofloxacin susceptibility test was found to be 19.5%, and among the E. coli isolates the prevalence of quinolone resistance was 19.8 %. 91 patients had underlying diseases and they were more frequently complicated by urosepsis. The ages and the prevalence of azotemia were not significantly different between two groups. Quinolone-resistant group had a lesser frequency of urosepsis compared with quinolone-susceptible group. Diabetic patients were more frequently quinolone-susceptible than quinolone-resistant. CONCLUSION: Although the prevalence of quinolone resistance is rather high in community-acquired APN based on the in vitro susceptibility test, the choice of quinolone antibiotics seems to be valid for the empirical therapy.