We performed a retrospective study comparing computed tomography (CT) and DMSA-Tc-99m scintigraphy (DMSA) in the evaluation of renal disease in acute pyelonephritis. Thirty-four 39 +/- 20 year old female patients were studied DMSA scintigraphy was performed 11.4 +/- 5.8 days after infection onset. Planar posterior views were obtained 4 to 6 hours after injection of 80 MBq of Tc-99m-DMSA. Renal uptake was also quantified after kidney depth had been measured by ultrasonography. CT was performed with and without bolus injection of contrast medium 15.5 +/- 10.6 days after infection onset DMSA images' were read independently by three blind NM doctors, using three posterior views. Cortical renal defects were evaluated on three different renal segments an the top. middle and bottom. CT films were also read by three expert physicians. Thirty-two out of 34 patients had renal cortical defects on DMSA and 17 had only CT consistent with pyelonephritis. Cortical defects were bilateral in 22 out of 32 patients and in three out of 17 CT studies. Bilateral and left kidney lesions on CT (five cases) were also present on DMSA. Four out of 12 patients with isolated right kidney abnormalities at CT scan showed defects limited to the right kidney on DMSA, but eight had bilateral defects. After exclusion of two patients with renal failure, ten normal kidneys had a 22.9 +/- 5.4% uptake that was significantly different from that of damaged kidneys (18.8 +/- 7.4%), p < 0.05. In 204 renal segments, 29 showed a concordant defect on CT and DMSA, and 127 were normal on both scans. Eighteen showed defects on CT but did not on DMSA, 27 were CT negative but DMSA positive, with three DMSA false positives due to renal cysts. We concluded that DMSA cortical renal scan appears to be more sensitive than CT to evaluate renal damage in patients with acute pyelonephritis.