A prospective study evaluated the utility of renal computed tomography (CT) and ultrasonography in 35 patients hospitalized for treatment of urinary tract infection. Renal computed tomograms were abnormal in 18 of 28 patients with acute pyelonephritis and three of four patients with urosepsis, showing findings consistent with pyelonephritis in 17 patients and intrarenal abscess or focal bacterial nephritis in four patients. Renal sonograms were abnormal in only eight patients, showing findings compatible with pyelonephritis in four and intrarenal abscess or focal bacterial nephritis in the other four. Flank tenderness was absent in only four patients with CT findings of pyelonephritis, of whom three were diabetic. We therefore found that (1) renal CT is a sensitive test for acute upper urinary tract infection, (2) ultrasonography detects focal bacterial nephritis and abscesses but is insensitive to uncomplicated upper urinary tract infection, and (3) painless pyelonephritis may be more common in patients with diabetes mellitus.