Observations on 13 patients with primary malignant fibrous histiocytoma of bone were reported. Included were nine male and four female patients, ranging in age from 6 to 81 years (mean, 44.8 years). Pain was the most common complaint. The interval from the first symptom to the initial treatment varied from 2 months to 20 years. Seven tumors arose in the knee region. Roentgenologically, most of the lesions presented with an osteolytic and destructive appearance. Histologically, highly variable morphologic features existed. The storiform-pleomorphic pattern was found in every tumor, although it was not necessarily pathognomonic for malignant fibrous histiocytoma of bone. Surgery, radical or incomplete, was the primary treatment for all but one patient. Lymph node metastasis was present in three. Five patients died of the disease from 3 to 79 months after the diagnosis (mean, 28 months), all exhibiting metastasis to the lung. Gaucher's body-like structure demonstrated in the electron microscopic study shows the histiocytic quality of malignant fibrous histiocytoma of bone.