A kidney tumour in a 36 year old female is described. The tumor was soft, of gray-brown color, size 12 x 11 cm. Histological, immunohistochemical and ultrastructural findings have demonstrated the signs of biphasic epithelial and mesenchymal cell differentiation. These data allowed the authors to define this tumor as a renal carcinosarcoma. Histogenesis of this tumor and differential diagnosis with other malignant renal tumors are discussed.