A malignant nerve sheath tumor of the acoustic nerve was found in a 54-year-old man whose right acoustic nerve signs had shown gradual worsening during the previous five years. No stigmata of von Recklinghausen's disease were noted. The tumor at autopsy extensively involved the brain stem, cerebellar hemisphere, tentorium of cerebellum, and cerebral occipitotemporal lobe on the right side. Histologically, the tumor was characterized by extreme hypercellularity, pleomorphism, and high mitotic activity. Ultrastructurally, there was an external lamina around the interdigitated cytoplasm of the tumor cells. Long-spacing collagens and junctional devices were also noted. This case is, to our knowledge, the first reported of a malignant nerve sheath tumor in the acoustic nerve.