A clinicopath-logic and electron microscopic study was performed on 35 cases of primary malignant peripheral nerve tumors, among which 12 developed in association with neurofibromatosis (von Recklinghausen's disease) and further 11 in keeping with anatomically discernible nerves in patients without neurofibromatosis. Depending upon the histologically predominant pattern, these tumors were subdivided into three groups: 23 compact spindle cell, 6 myxoid, and 6 epithelioid varieties of the tumor. The common ultrastructures in three of the 35 tumors were as follows: 1) The cell membranes manifested characteristic infoldings and lamellar configuation. 2) The tumor cell surfaces were coated by occasional basal lamina or homogeneously electron-dense membranous material. 3) The cytoplasms contained well-developed organelles and a few neurosecretory-type granules. Differential points from other soft-tissue sarcomas were briefly discussed on the histologic basis.