A case of sarcomatoid carcinoma of the renal pelvis is reported. The neoplasm showed polypoid configuration and was composed exclusively of plump, spindle and pleomorphic cells. Light microscopy did not reveal any epithelial differentiation of the neoplastic cells. Immunoperoxidase staining for keratins and for epithelial membrane antigen was strongly positive in the spindle elements and showed the epithelial nature of the proliferation. The present case suggests that pleomorphic tumors, when occurring in visceral organs, should be carefully sampled and immunocytochemical markers for epithelial and sarcomatous differentiation studied, before diagnosis of carcinosarcoma or sarcoma may be accepted.