Wilms' tumor in a multilocular cyst of the kidney (cystic Wilms' tumor).

A 2-year-old boy had a Wilms' tumor of the left kidney that appeared during physical examination as an abdominal mass and on gross examination as a multilocular cyst. Multilocular cyst of the kidney is a rare but distinctive congenital lesion. Its true biologic behavior is unknown. Although simple nephrectomy is sufficient for most lesions, more aggressive procedures are recommended for those lesions that contain Wilms' tumor (cystic Wilms' tumor).