The International Reflux Study in Children was set up to compare prospectively the outcome of medical or surgical management of children with grade III or IV vesicoureteral reflux and a history of symptomatic urinary tract infection. Development of new radiological scars was the main end point. Of the 306 children randomized, 302 (153 medical, 149 surgical) were available for radiological follow-up at 5 years. New scars had developed in 19 medically and 21 surgically treated children. Among 223 patients (113 medical, 110 surgical) who continued follow-up with urography at 10 years, only 2 further new scars developed. Overall, 47 new scars were acquired in 42 patients (20 medical, 22 surgical), 25 of them in children with unscarred kidneys at entry (13 medical, 12 surgical). New scars occurred mostly in children under 5 years of age and were observed more frequently in children with grade IV than grade III reflux. We conclude that with careful management, only a small proportion of children with severe reflux developed new scars and rarely after the first 5-year follow up period, and that there was no difference between children treated medically or surgically.