Soft tissue sarcoma with additional anaplastic components. A clinicopathologic and immunohistochemical study of 27 cases. This clinicopathologic study concerns 27 cases of "dedifferentiated" soft tissue sarcoma (DSTS), including 14 liposarcomas, six leiomyosarcomas, five chondrosarcomas, and two rhabdomyosarcomas. In addition, the authors conducted an immunohistochemical survey of 23 cases and an electron microscopic examination of three. The findings were compared with observations of 32 cases of de novo malignant fibrous histiocytoma (MFH). All tumors contained additional distinct anaplastic portions indistinguishable from MFH under conventional light microscopy, ultrastructurally, and in cases of immunoreactivity for alpha-1-antichymotrypsin and alpha-1-antitrypsin and on lectin histochemical findings for ricinus communis agglutinin and concanavalin agglutinin. The desmin reactivity present in anaplastic portions of 14 DSTS and in eight de novo MFH is taken to mean that myofibroblasts are present in these tumors. The anaplastic components of DSTS are presumed to represent the proliferation of another clone of undifferentiated mesenchymal cells that fail to differentiate along any specific lineage other than fibroblast-like cells, histiocyte-like cells, and myofibroblasts. Nineteen patients died of tumor and four are alive and well 1.6, 1.7, 2.1, and 5.2 years after the initial treatment, respectively. 