Ten cases of duplex kidney with ectopic ureterocele were detected by ultrasound in the recent four years. There were eight girls and two boys. One case was found to have bilateral ectopic ureterocele, the others were unilateral. Urinary tract infection was found in five cases, urinary dribbling was told in one case, and the other four cases were asymptomatic. The age distribution ranged from newborn to 15 years. Six cases received surgery, and the other four cases were followup at O.P.D. In these cases, the sonography could identify the hydronephrotic nature of the upper pole moiety. Dilatation of the upper pole ureter could be demonstrated. A sonolucent intravesical cystic mass also could be outlined. In this series, five cases with residual parenchyma were assessed by ultrasonography, and two cases with pus formation in the upper pole collecting system were detected. On intravenous urogram, the affected upper pole moiety was not or poorly visualized in these cases, thus produced some diagnostic challenges. Ultrasonic examination is an excellent method to identify a fluid-filled cystic structure, and can provide a simple and non-invasive technique for confirmation of ectopic ureterocele, which is independent of renal function. Therefore, it has become an important modality in the detection, diagnosis and followup of this anomaly.