Dedifferentiated chondrosarcoma is a highly malignant variant of chondrosarcoma. Approximately 11 per cent of chondrosarcomas can be expected to dedifferentiate into more anaplastic lesions. In this report, we analyze the clinicopathological features and treatment of seventy-eight lesions of this type. The ages of the patients ranged from nineteen to eighty-two years (average, 54.6 years). The cartilaginous precursor was central in most patients. Eleven of the lesions developed in the site of a previously resected low-grade chondrosarcoma. Dedifferentiation was from low-grade chondrosarcoma to osteosarcoma in forty-two patients, to fibrosarcoma in thirty-three, and to malignant (fibrous) histiocytoma in three. Perforation of the cortex and a soft-tissue mass were found in most of the patients. Widespread metastatic disease within two years after resection was a frequent finding. The over-all five-year-survival rate was 10.5 per cent. Any potential for a "cure" is related to early diagnosis and adequate surgical treatment by amputation or resection.