The term Vilyuisk encephalitis refers to a clinically distinct and geographically isolated form of encephalitis that has, for at least several generations, been recognized among the population of Vilyuisk and neighboring regions of the Yakut Republic in the U.S.S.R. Clinically, it can occur in an acute or chronic form, in which syndromes of diffuse meningoencephalitis, dementia-schizophrenia, and amyotrophic lateral sclerosis are recognized. About one-third of the patients progress from the acute to the chronic stage after an interval of weeks to months, but the chronic stage may follow repeated exacerbations of acute symptoms, or appear without any recognized antecedent acute stage. Examination of tissues taken at autopsy has revealed widespread inflammatory-degenerative changes in the nervous system, especially in the cerebral cortex and basal ganglia. The possible role of rodent or avian hosts in the transmission of the disease is under study; migratory birds traveling between Yakutsia and the western Pacific are especially intriguing because of the foci of amyotrophic lateral sclerosis in Japan and the Marianas. A virus related to the mengo- and encephalomyocarditis group has been isolated from animals inoculated with tissue from patients with Vilyuisk encephalitis, but the same virus is known to be latent in wild and laboratory rodents. Occasional clustering of cases in the same family, often after long intervals, and the interval between acute and chronic forms of the illness rather suggest that it may be due to an agent within the group of slow, latent, and temperate viruses.