Ninety-one children 1 week to 10 years old with culture-documented febrile urinary tract infection were evaluated with renal sonography and renal cortical scintigraphy by using 99mTc-labeled dimercaptosuccinic acid (DMSA). On the basis of previous experimental studies, DMSA scintigraphy was used as the standard of reference for the diagnosis of acute pyelonephritis. The DMSA scans showed changes consistent with acute pyelonephritis in 63% (57/91) of the patients. Sonograms showed changes consistent with acute pyelonephritis in 24% (22/91) of the whole group and in only 39% (22/57) of the patients with scintigraphically documented acute pyelonephritis. Pertinent sonographic findings were areas of increased cortical echogenicity in 14 patients and decreased echogenicity in eight, including three patients with renal abscesses. Dilatation of the renal collecting system was noted in nine patients and renal enlargement was noted in three. We conclude that renal sonography is a relatively insensitive test for the detection of acute inflammatory changes of renal cortex. Therefore, it should not be used as the primary imaging technique for the diagnosis of acute pyelonephritis. However, sonography is a useful technique in evaluating the nature of the defects seen on the DMSA scan and in detecting obstructive uropathies that may be associated with urinary tract infections.