The relative frequency of aseptic meningoencephalitis (AME) was compared in populations of diverse origin, A countrywide search of Israel during 1969-1970 disclosed 1350 cases who fit strict diagnostic criteria. The average annual incidence was 21.6 per 100000 population. The total incidence was similar in Afro-Asian, Euro-American and Israeli Jewish groups but among Israeli Arabs, the incidence was apparently lower. Age-specific incidence showed a peak in infants under one year of age among Arabs and Afro-Asian Jews whereas Euro-Americans and Israeli Jews had a peak incidence at 5-9 years. Larger family size among Arabs and Afro-Asian Jews might account for the higher incidence in infants. Age-specific incidence may be a better index than total incidence of important differences in AME among various ethnic groups.