Purpose: A complete duplicated collecting system complicated with an ectopic ureter or ureterocele results in renal damage due to a ureteral obstruction or reflux. The clinical outcome of ureteropyelostomy in complicated complete duplicated collecting system was evaluated. Materials and Methods: 20 children received a ureteropyelostomy for either an ectopic ureter or ureterocele with a complete duplicated collecting system anomaly. The median age was 3.2 months. Of the children, 11 had ureteroceles including 7 cecoureteroceles, and 9 had ectopic ureters. The associated anomalies were 3 UPJ obstructions of the lower pole and 9 VUR. All received a ureteropyelostomy. The lower pole ureter was used as the common ureter except in 2 cases with a lower ureter in poor condition. A lower pole pyeloplasty was combined in 3 cases with UPJ obstructions of the lower pole. A distal ureteral stump was left open except in the cecoureterocele in order to prevent VUR. Their clinical courses were followed up with an ultrasonogram and 99mTc-DMSA scan. The median follow-up after surgery was 17.8 months. Results: A reduced upper pole hydronephrosis was observed in 19 patients and an improvement in the differential renal function was noted in 17 patients on the follow-up studies. All ureteroceles had either disappeared or were reduced in size. 7 patients showed an intermittent asymptomatic bacteriuria and 1 patient showed symptomatic UTI who required an ureteroneocystostomy during the follow-up. Conclusions: A ureteropyelostomy is recommended as a first-line treatment modality for patients with a complicated complete duplicated collecting system owing to the lower chance of secondary surgery, the prevention of deterioration in the bladder function and the possibility of early correction.ope