We report for the first time a "giant megaureter" causing symptomatic compression of the contralateral ureter in a 15-year-old girl. Extrinsic compression of the ureter by a contralateral giant hydronephrotic kidney, although rare, has been reported most commonly because of congenital pelviureteric junction obstruction. In this case, the possible etiology of megaureter, the differential diagnosis, and the clinical implications are discussed.