Thirty myxofibrosarcomas are described. These malignant soft tissue tumors are characterized by a mucoid and nodular appearance, a coarse plexiform capillary pattern and they are mostly seen subcutaneously (26 of 30) in the extremities (24 of 30) and trunk (4 of 30) of elderly people. Histochemical studies, comprising staining with Alcian blue and toluidine blue at different pH''s with and without preceding digestion with testicular hyaluronidase and the Scott technique, indicated the presence of hyaluronic acid but not sulfated glycosaminoglycans as chondroitin sulfates. Myxofibrosarcoma is believed to belong to the general category of fibroblastic and histiocytic malignant soft tissue tumors. The median diameter of the tumors was 7 cm. They were divided into 4 grades according to cellularity, cell atypia and mitotic activity. The grade III and IV tumors showed pronounced atypia, often with bi- and multinucleated giant tumor cells and occasionally with giant cells of Touton''s type, suggesting a relationship to malignant fibroxanthoma. All of the patients were treated surgically and 1 also received pre- and postoperative irradiation. None of the 2 grade I myxofibrosarcomas recurred, while 2 of 7 grade II tumors, 6 of 10 grade III tumors and 7 of 11 grade IV tumors recurred once and up to 9 times. Metastases appeared in 7 of 30 patients; grade I tumors were not seen in any of these cases. By the time of follow-up after intervals ranging from 1 mo.-27 yr, 14 patients died; 6 of these died postoperatively or of intercurrent disease. The differential diagnosis between myxofibrosarcoma and other myxoid soft tissue tumors is discussed.