BACKGROUND: Acute pyelonephritis in women is one of the most common infections within the community; some patients also suffer from related bacteremia and renal abscess. The predominant pathogen in acute pyelonephritis is Escherichia coli and the changes in antimicrobial resistance over time is a very important factor in the choice of effective and economic antibiotics. MATERIALS AND METHODS: We investigated clinical features and antibiotic sensitivities of 577 organisms isolated from the urine cultures of 577 patients, admitted to Catholic University St Vincent's Hospital for community-acquired acute pyelonephritis from January 2001 to December 2006. We analyzed the patterns of antimicrobial resistance of urinary isolates and the clinical courses of the patients. RESULTS: Patients demographics revealed a mean age of 51, (age:16 to 91), with bacteremic patients representing 30.8% of patients and renal abscess patients representing 5.9% of the group. Sixteen (4.2%) of 382 in the pyelonephritis group and five (3.1%) of 161 in the bacteremia group revealed clinical manifestations of therapeutic failure such as persistent fever and pyuria. The mean time to defervescence was 44.6 h for the pyelonephritis group, 76.4 h for the bacteremia group and 91.2 h for the renal abscess group. Among the 577 isolates, 554 isolates were E. coli, 10 were K. pneumoniae, three were S. saprophyticus, three were Proteus mirabilis. two were K. oxytoca, and two were Enterobacter aerogenes. Among 554 E. coli, the rates of susceptibility to ampicillin was 38.3%; to sulfamethoxazole 62.1%; to gentamicin 81.3%; to ciprofloxacin 86.3%; to cefuroxime 97.3%; to amikacin 98.7%; to cefotaxime 99.5%. CONCLUSION: In hospitalized patients, initial intravenous treatment with an aminoglycoside or a second -generation cephalosporin, and then switch to oral first, second-cephalosporin, amoxicillin and sulfamethoxazole is recommended. In vitro resistance to fluoroquinolones appears to be increasing, and therefore close monitoring of antibiotic susceptibility patterns in isolates of urinary tract infections and the use of fluoroquinolone-sparing agents are required.