Two cases of duplex kidney with ectopic urethral ureter are reported. Using abdominal ultrasound, the poorly functioning upper pole pelvis of the involved kidney was found to be sonolucent which may have been misinterpreted for a simple renal cyst. The corresponding ureter was dilated and drained into the proximal urethra. Ultrasound is an ideal imaging modality in the assessment of children with a duplicated renal system and may provide detailed information about this structural anomaly, such as the exact insertion of the ectopic ureter.