We describe a case of congenital giant megaureter in a 16-year-old female. She presented with a 5-day history of abdominal distention, right flank pain and tenderness. Right pyelonephritis was suspected. Computerized tomography showed a large cystic abdominal mass with no appreciably functioning left kidney causing secondary compression of the contralateral right ureter. A left upper nephroureterectomy was performed, draining over 3.5 L of fluid. Our experience suggests that congenital giant megaureter should be considered in the differential for pediatric patients presenting with a cystic abdominal mass.