The results of neurological, pathomorphological and clinic-genealogical studies of vilyuisk encephalitis (VE), a chronic progressive disease of CNS, are presented. The disease is spread in Yakutija in Vilyui river basin (mainly in Vilyuisk and Kobyaisk regions) exclusively among native population and is known there since the middle of the last century. The results obtained suggest that VE (despite the existing opinion on its natural-focal virus etiology) is an ethnic hereditary disease with autosomal recessive type of transmission, is spread in a limited Yakut population by an ancestor effect and gradually gets out of the limits of this population at the expense of migration. According to its clinical symptoms and pathomorphological substrate of this main neurological syndrome (peculiar lower spastic paraparesis as a result of systemic ascending degeneration of lateral corticospinal tract of spinal cord), VE belongs to hereditary heterogenous group of diseases which are referred to Strümpell's spastic paraplegia. VE is different from the diseases of this group by the occurrence of cases with very rapid progressing and characteristic brain damages. To prevent further VE accumulation, it is necessary to concentrate efforts on biochemical and neurological studies in search of reliable tests for early detection of heterozygotes. Taking into account the hereditary nature of VE, its ethnic limitations and nosological self-dependence, it is expedient to designate the disease by its local name "bokhoroor".