Ewing's sarcoma of the kidney.

Primitive neuroectodermal tumors (PNETs) are small, round-cell tumors of neural crest origin classically found in the central nervous system (CNS) but more recently characterized in the periphery. Peripherally located PNETs (pPNETs) are members of the Ewing's sarcoma family of tumors (EFTs). Renal localization of pPNETs is very rare, is found in young adults, and is characterized by an aggressive clinical course and poor prognosis.