Eighteen patients with chondrosarcomas showing dedifferentiation were reviewed. Thirteen of the 14 patients in whom there was follow-up died of their tumors, with an average survival of six months. The average age was 61, and the most common location was the distal femur. Eight patients presented with pathologic fractures. Radiographically the tumor appeared as a destructive are with an associated heavily calcified lesion. This corresponded to combined histologic patterns of high grade sarcoma, usually with features of a malignant fibrous histiocytoma, and a low grade chondrosarcoma. This lesion must be distinguished from a primary malignant fibrous histiocytoma of bone in view of the different prognostic implications.