Recurrent epidemics of encephalitis in Nagaland, a North-Eastern State of India, following its first appearance in 1985, were investigated both epidemiologically and virologically. Although, no viral agent could be isolated from any of the clinical samples and mosquitoes, detection of JE specific IgM antibodies in many of the CSF and acute blood samples, together with presence of HI and CF antibodies to JE antigen in a number of acute and convalescent sera established the etiologic role of JE virus in this region. A total number of 83 clinically diagnosed cases could only be investigated virologically between 1985-89, where evidence of JE could be established in 34 (40.9 per cent) and flavivirus (including JE in majority) in 17 (21.5 per cent) cases. A limited serological survey among the close contacts of the victims in 1985 revealed JE antibody in 26.6 per cent of them. Analysis of the epidemiological and serological findings of different years revealed that while the outbreaks of 1985, 1987 and 1988 were due to JE infection, the episodes of 1986 and 1989, on the other hand, had chiefly features of high and prolonged fever with limited number of CNS involvement of undetermined origin, where the possibility of malarial infection has been a suspect apart from JE etiology.