A series of 105 children with vesicoureteral reflux into completely duplicated collecting systems was studied retrospectively. Of the children 64% were treated surgically, with two-thirds undergoing an ablative procedure and one-third undergoing common sheath ureteral reimplantation. The remaining 36% of the children were managed nonoperatively and followed for up to 11 years. Among these cases 89% of the kidneys had reflux into the lower pole moiety only and 11% had reflux into the upper and lower pole moieties. The spontaneous rates of reflux resolution were 85% for grades I and II, and 36% for grade III reflux during a median observation period of 40 months. These figures suggest that the spontaneous resolution of mild to moderate reflux into completely duplicated collecting systems is similar to that of identical degrees of single system reflux during a comparable observational period.