Reye syndrome in siblings was seen in three of 85 families; the incidence of RS in these family groups appears to exceed that of the general population. The interval between development of RS in the first and second siblings was two to 11 days and related to the incubation period of the initial viral infection. In five of the children this infection was chickenpox and in two, an unspecified upper respiratory illness. To assess the role of genetic factors, HLA typing was performed on these siblings; a common genetic marker indicating susceptibility to RS was not identified. All families resided in rural and suburban areas; exposure to a common environmental toxin was not identified.