PURPOSE: We determined the impact of vesicoureteral reflux (VUR) on the size of renal lesions in children after an episode of acute pyelonephritis. MATERIALS AND METHODS: A total of 161 children (mean age 2.44 years) with acute pyelonephritis were studied. All had renal lesions on dimercapto-succinic acid scintigraphy done at admission to the hospital. A second dimercapto-succinic acid scan was performed at 3 months. Voiding cystourethrography was done at 6 weeks and VUR was graded I to V. For each renal unit layouts of renal lesions were drawn, and the damage surface was calculated and reported for the total surface of the kidney. RESULTS: Mean size of acute lesions and scars increased with severity of reflux (p <0.0001), with an important overlap of individual values. Mean size of renal scars in the group of renal units with acute lesions was 5.8% +/- 8.5% in patients without VUR, 9.9% +/- 7.3% in those with grade I reflux, 7.7% +/- 11.0% in those with grade II reflux, 17.7% +/- 14.7% in those with grade III reflux and 17.4% +/- 27.7% in those with grade IV reflux (p <0.001). The size of renal lesions decreased significantly with time. The rate of regression of lesions decreased with increasing reflux. When analyzed according to 3 age groups sizes of scars increased significantly with age. CONCLUSIONS: VUR has an impact on the size of renal lesions after an episode of pyelonephritis. Children with a grade III or IV reflux are more likely to have larger renal scars. On the other hand, acute lesions of important size may develop even in the absence of VUR.