To obtain data concerning their histogenesis, 23 benign solitary schwannomas (including five cellular variants), 10 neurofibromas, and 10 malignant peripheral nerve sheath tumors were studied by electron microscopy. The results confirm previous findings that solitary schwannomas (so-called neurilemomas) are composed principally of cells showing features of differentiated Schwann cells. The principal cells of neurofibromas on the other hand did not resemble differentiated Schwann cells. The predominant cell type in six cases was indistinguishable from normal perineurial cells. Schwann cell-axon complexes seen in three cases may have represented entrapped normal structures or perhaps one component of a hamartomatous tumor. In contrast to the benign neoplasms the cells of the ten malignant peripheral nerve sheath tumors were in general poorly differentiated. When differentiated, they shared some features with Schwann and perineurial cells. Cell forms intermediate between them and fibroblastic cells were also identified. These findings indicate that schwannoma and neurofibroma are distinct entities. In the authors' experience schwannomas rarely undergo malignant change. For this reason and because it is unclear whether malignant tumors of peripheral nerves arise from more than one sheath cell, the designation of malignant peripheral nerve sheath tumor (PNST) is preferable to malignant schwannoma.