A total of 532 children, 401 from Europe and 131 from the United States, was recruited into an international multicenter study comparing the results of medical and surgical treatment of children with international grade III or IV vesicoureteral reflux and urinary tract infection. Patient age was less than 11 years, glomerular filtration rate was greater than 70 ml. per minute per 1.73 m.2 and there was no obstruction, renal malformation, previous urinary tract surgery or neuropathic bladder. A total of 80 children had less than grade III vesicoureteral reflux on a second pre-entry cystourethrogram required by the European protocol, and they were followed separately as a sideline group. At entry the age distribution, history of urinary tract infection and proportion of children with grade IV vesicoureteral reflux were similar in Europe and the United States. Of the European and United States children 48% and 54% had renal scarring, and 17% and 14% had parenchymal thinning, respectively. In each group renal length and planimetric area were normal in two-thirds and 5% had small kidneys (<2 standard deviations from normal). Differences included sex distribution (24% boys in Europe and 11% in the United States group), the proportion of children in whom vesicoureteral reflux was previously known (18% Europe and 69% United States) and the proportion of children with bilateral reflux (77% Europe and 57% United States). Randomization and stratification for treatment were successful.