OBJECTIVE Emphysematous urinary tract infection (EUTI) is a rare and potentially life-threatening condition that requires prompt evaluation and management. This study was conducted to ascertain the clinical features and prognostic factors of EUTI. METHODS Patients diagnosed with EUTI radiologically and treated at the Kaohsiung Medical University Hospital, Kaohsiung, Taiwan, from March 2001 to February 2007 were evaluated. The patients' demographic and clinical characteristics, laboratory data, treatment, and outcomes were analyzed retrospectively. RESULTS Of 31 patients enrolled, 16 had emphysematous pyelonephritis (EP) and 15 had emphysematous cystitis (EC) classified according to the imaging findings. The symptoms and signs of fever, chills, flank pain, and percussion tenderness at the costovertebral angle were significantly greater among patients in the EP group than in the EC group (p = 0.029, p = 0.009, p < 0.001, and p < 0.001, respectively). There were no statistically significant differences in the initial laboratory data except for C-reactive protein between the 2 groups (220.4 microg/mL vs 91.4 microg/mL; p = 0.001). Escherichia coli was the most commonly isolated organism. The overall mortality rate was similar in both groups. Significant differences in renal function and hematuria were seen between the patients who died and the survivors in the EP group (p = 0.004 and p = 0.027, respectively), but these were not noted in the EC group. CONCLUSIONS There was no significant clinical feature suggesting the presence of EC. The clinical features of EP were similar to uncomplicated pyelonephritis. Impaired renal function and hematuria were poor prognostic factors for patients with EP, but not for patients with EC.