PURPOSE: For prevention of renal scar, early diagnosis and treatement of acute pyelonephritis are important in children with febrile urinary tract infection. But it has been difficult to differentiate acute pyelonephritis from lower urinary tract infection. We have performed this study to evaluate the diagnostic value of 99mTc-DMSA renal scan and to clarify the relationship among acute pyelonephritis, vesico-ureteral reflux and renal scar. METHODS: We have studied 100 patients with febrile urinary tract infection, from September, 1993 to June, 1995 at the Pediatric department of Ewha Womans University, Mok-dong hospital. Acute phase reactants, causative organism, renal ultrasound, 99mTc- DMSA renal scan on the acute stage, voiding cystouretherogram after 1-2 weeks of treatement, and follow-up 99mTc-DMSA renal scan were evaluated. RESULTS: Ther results are as follows; 1) 57 patients (57%) had defect on 99mTc-DMSA renal scan and diagnosed as acute pyelonephritis. 2) Acute phase reactants, renal ultrasound had low accuracy. 3) VUR was found in 31 (56.1%) of 57 patients with defect which was significantly higher than the 4 (9.3%) of 43 patients without defect on 99mTc-DMSA renal scan. 4) On the follow up 99mTc-DMSA renal scan, 34.8% of defects recorvered, and 65.2% of defects developed renal scar. CONCLUSIONS: 99mTc-DMSA renal scan is a valuable method for early diagnosis of acute pyelonephritis in children with febrile urinary tract infection. It would contribute to evaluate the relationship between acute pyelonephritis, vesico-ureteral reflux and renal scar.