We report an extraordinarily rare case of a 17-year-old male with an extraskeletal Ewing's sarcoma (ESS) of the kidney and a massive thrombosis involving the inferior vena cava (IVC), from the iliac axis to the right atrium. This onset resembled renal cell carcinoma (RCC), although histological examination revealed it was an extraskeletal Ewing's sarcoma/peripheral neuro-ectodermal tumor (EES/PNET). EES/PNET should benefit from neoadjuvant chemotherapy to reduce the risk of metastasis and of recurrent disease due to delay in suitable treatment. Therefore, in the presence of a renal mass with tumor extension of IVC, it is reasonable to bear in mind that other tumors, apart from RCC, could occur. In such cases, a US or CT-scan guided biopsy could be useful.