Adult Wilms' tumor mimicking hemorrhagic renal cyst.

A 36-year-old man presented with macroscopic hematuria associated with right flank pain. Examination of the patient revealed a cystic mass in the right kidney. Because the mass had increased in size, enucleation of the mass was performed. Histopathological findings revealed nephroblastoma, therefore, radical nephrectomy was performed. We believe the pathogenesis of the cystic formation to be a process in which a tumor that had developed in the pericalyceal region spontaneously ruptured, exuding urine into the perinephric space, forming a cystic mass. The patient is alive with no evidence of disease 24 months after the operation.