A 6-year-old boy who had been diagnosed as having Wilms tumor at the prior hospital was admitted to our hospital for investigation of a right flank mass and macrohematuria in October 1990. Magnetic resonance imaging showed Wilms tumor of the right kidney extending into the inferior vena cava. Right radical nephrectomy, lymphadenectomy and thrombectomy were done on October 30th, 1990. Pathological findings indicated clear cell sarcoma of the kidney. One week after the operation, radiotherapy and chemotherapy with actinomycin D (ACTD), vincristine sulfate (VCR), doxorubicin hydrochloride (DXR) and cyclophosphamide (CPM)) were started according to the protocol by National Wilms Tumor Study (NWTS) III. However, he died of bone metastases and pneumomycosis on October 13th, 1992. Clear cell sarcoma of the kidney (CCSK) is a rare tumor accounting for about 4-5% of Wilms tumors. This is the first report on CCSK with tumor thrombus in Japan.