A total of 321 children less than 11 years old with nonobstructive grade III or IV vesicoureteral reflux and with previous urinary tract infection was randomly allocated to medical or surgical treatment in the European branch of the International Reflux Study in Children. (Randomization was stratified for age, sex, grade of reflux, presence of renal scarring, interval since last urinary tract infection and treating hospital). The results of excretory urography are reported for 233 girls and 73 boys treated according to the random allocation, 89% of whom were followed for 5 years. After 5 years in the medical group (155 children) new renal scars were seen in 19 and new renal parenchymal thinning in 11. The proportions were almost identical among 151 children allocated to surgical treatment with 20 new scars and 15 new thinnings. Progression of established scars was also similar in both groups. However, the new scars developed sooner after surgery than during medical treatment. In 6 surgically treated children postoperative obstruction was followed by the development of new scars. In addition, 12 patients showed new scars approximately 6 months after successful surgery, while in only 2 children scars developed more than 6 months after surgery. In 11 children of the medical group new scars were seen more than 6 months after allocation. More new scars developed in the children with parenchymal thinning at entry (23%) than in those with scarred or normal kidneys at entry (10% each) (p < 0.05). The younger the patients at entry, the higher the frequency of new scars (less than 2 years 19.8%) 2 to 4 years 9.8% and 5 years or more 4.6%, p < 0.05).