Acute pyelonephritis causes hospitalization and is a commonly-ignored cause of death in geriatric patients. It has been well studied in young-adult populations but rarely in geriatric populations. The aim of our study was to analyze the characteristics of acute pyelonephritis in geriatric patients. The electronic admission records of a community hospital in northeastern Taiwan were retrospectively screened from July 1, 2003 to June 30, 2006. The basic characteristics, laboratory findings and infectious microorganisms of all subjects were evaluated. Sixty-five subjects (mean age 71.6 +/- 4.9 years; range 65-84 years) and 73 admission records contributed by them were enrolled. These 65 subjects, including one who died in hospital, were predominantly female (52 subjects; 80%). Twenty-two subjects (33.8%) had co-existing diabetes mellitus, 9 subjects (13.8%) had co-existing tumors, and 19 subjects (29.2%) had a history of intra-abdominal surgery. The admission records revealed right kidney involvement (52.1%), co-existing urolithiasis (50.7%) and admission to wards of internal medicine (57.5%). Urological procedures were performed on 20 (27.4%) of all 73 admission records. Escherichia coli was the most common infecting microorganism (19.2% of all records; 42.4% of records with positive microorganism culture). Hemoglobin < 10 g/dl was a significant predictive factor for both hospital stay > 7 days and serum creatinine > 2.0 mg/dl (p = 0.003 and 0.002, respectively). Positive microorganism culture was also a significant predictive factor for hospital stays > 7 days (p < 0.001). In our geriatric patients with acute pyelonephritis, low hemoglobin levels implied co-existing renal insufficiency and prolonged hospitalization. Positive microorganism culture also implied prolonged hospitalization.