To determine the important factors involved in the etiology of renal scarring we studied 37 children with renal scars seen at our hospital since 1965. This is the second largest series reported to date. Children who had neurogenic bladders or any structural abnormalities of the urinary tract other than vesicoureteral reflex were excluded. The study group included 36 girls and 1 boy. The average age at first detection of renal scars was 5.7 years. Acute pyelonephritic episodes, which were treated early and aggressively, infrequently led to renal scarring. However, the initial prolonged or poorly treated episode of acute pyelonephritis was followed invariably by the development of renal scarring. The severity of renal scarring was related to the grade of vesicoureteral reflux (p less than 0.05), although some scars did develop in the absence of reflux. Neither the shape and position of the ureteral orifice nor the ureteral tunnel length correlated with the severity of renal scarring. Treatment with prophylactic antibiotics may have lessened the severity of renal scarring (0.1 less than p less than 0.2) but treatment with reimplantation surgery did not appear to alter the course of renal scarring. This study suggests that the key to the prevention of renal scarring is the early and aggressive treatment of acute pyelonephritis.