A previously healthy middle aged man died following a 6 month illness which presented with middle ear symptoms, apparently resolved, and then 2 months later manifested as encephalitis. The illness was characterized initially by depression and intellectual deterioration. No family member or working associate was affected. The clinical diagnosis of viral encephalitis was confirmed by brain biopsy but no virus was isolated in the laboratory. Numerous intracisternal toroidal virus-like particles were demonstrated by electron microscopy in the perikarya and dendrites but not in glia. The particles resemble, but are not identical to, the oncornaviruses associated with spontaneous and induced murine neoplasms. The resemblance of these structures to the intracisternal toroidal type "A" virus of murine leukemia is noted and other possible causes for this atypical meningoencephalitis are discussed.