The clinical and pathologic findings of 12 cases of malignant fibrous histiocytoma of the soft tissues are presented. The mean age of the patients (eight males and four females) was 64 years, and there was a strong predilection for localization in the extremities (10 cases). The lesion is of rather low-grade malignancy, with metastases in only a minority of cases (three cases) but with nonetheless a marked tendency for local recurrence (nine cases). Histologically, the essential feature of the tumor is its biphasic composition with both histiocytic and fibroblastic components, the latter showing a characteristic storiform pattern. In addition, a variable number of xanthomatous cells and multinucleated giant cells were observed. Ultrastructural studies confirmed the presence of the aforementioned cells and in addition revealed a primitive mesenchymal cell. It is suggested that this cell may be a primitive histiocyte from which the two main cellular components of the tumor arise.