Acute pyelonephritis (APN) is a frequent disease, but diagnostic approach, evolution into abscesses, and indication to hospitalization are still open problems. We have made a retrospective analysis of APN cases observed in our hospital. We identified 58 patients (pt) and selected 52 of these who presented fever and loin pain at the onset (31 were hospitalized in Nephrology and 21 in other units). Urine culture was positive in 11/48 cases (22.9%), blood cultures in 3/26 cases (11.5%) (Escherichia coli). Renal sonography was normal in 20/48 cases (41.6%) and suggestive for APN in 23/48 cases (47.9%). CT with contrast medium was normal in 9/28 cases (32.1%) and positive in 19/28 cases (67.8%), with evidence of unique or multiple hypodense areas; abscesses were found in 8 patients (28.5%). No statistically significant differences were found between patients with positive or negative CT as regards fever, leukocytosis, ESR, CRP, CRP at 20 days, urinary leukocytes, urine culture, duration of symptoms before hospitalization. Moreover no differences were found between patients with and without abscesses. CT was performed more frequently among patients hospitalized in Nephrology than among patients hospitalized in other services (24/31-77.4%-vs. 4/21-19%-, p = 0.05). NMR was abnormal in 6/9 cases. A radiographic documentation of APN was obtained in 61.53% of patients with clinical diagnosis of APN. Of these, only 18.7% had positive urine culture. In conclusion, our data suggest that demonstration of urine infection is not necessary for APN diagnosis, when clinical and/or radiologic diagnosis of APN has been made. Evolution into abscesses is frequent and not easily susceptible on clinical ground; for this reason we think it is advisable to perform CT or NMR systematically. Differences in clinical behavior between different units suggest the need for diagnostic guidelines.