PURPOSE We reviewed our experience with ureteroureterostomy as definitive treatment for vesicoureteral reflux or obstruction associated with ureteral duplication to determine the efficacy and morbidity of this procedure and identify factors that affect outcome. MATERIALS AND METHODS We performed a total of 100 ureteroureterostomies in 94 children with an average age of 28 months during the 23-year period ending May 1999. Four patients (4 kidneys) failed to return for postoperative evaluation. Followup was 2.5 to 206 months (mean 33). Indications for surgery were vesicoureteral reflux in 53 cases, obstructing ureterocele in 19, ureterocele with upper pole reflux in 4, an ectopic obstructed upper pole ureter in 18 and other in 2. In 23 kidneys ureteroureterostomy was combined with reimplantation of the recipient ureter. Surgery was considered successful when postoperative imaging revealed no reflux or obstruction and a patent ureteroureterostomy anastomosis. RESULTS Ureteroureterostomy with or without ureteroneocystostomy was successful for treating 94% of kidneys, including 51 of 53 with reflux, 21 of 23 (91%) with ureterocele and 17 of 18 (94%) with an ectopic obstructed upper pole ureter. Significant ureteral size disparity, defined as a donor ureter diameter greater than 2-fold that of the recipient ureter, was noted in 69 kidneys. The most common complication (13 patients) was prolonged output from the Penrose drain. However, this complication did not result in a failed procedure in any case. CONCLUSIONS Ipsilateral ureteroureterostomy is safe and efficacious for treating abnormalities associated with ureteral duplication. A significant discrepancy in ureteral size does not preclude ureteroureterostomy.