Thirty-five cases of primary malignant fibrous histiocytoma of bone are reported. Twenty of these cases were collected from a retrospective analysis of other malignant bone tumors. The age range was from 11 to 69 years; the average age was 34 years. The tumor occurred most commonly in the distal femur and proximal tibia. The distinguishing histologic feature was a storiform arrangement of spindle cells. The differential diagnosis included fibrosarcoma, osteogenic sarcoma, malignant giant cell tumor, malignant lymphoma, and metastatic carcinoma. Follow-up of at least three years was available in 21 cases. Of these, nine patients were alive and free of metastases three and one-half to 12 years after treatment. Two were alive with solitary metastases at three years, and 10 patients died between three months and three years after treatment. In four cases the lesions were multicentric at the time of diagnosis and in four cases were associated with bone infarction. This tumor must be recognized as an important complication of bone infarction and should be suspected when a patient with a known history of bone infarction develops a change in symptoms. Because the prognosis of this tumor is significantly better than that in those tumors with which it had been previously grouped, and in view of its association with bone infarction, it deserves to be maintained as a distinct clinicopathologic entity. Amputation is the treatment of choice.