Data from linked birth and death certificates from the State of Oregon were used to determine the monthly distribution of deaths from sudden infant death syndrome (SIDS) for the years 1976 through 1984. The monthly number of SIDS deaths increased in winter and decreased in summer, when schools usually are not in session. Absences from kindergarten were determined from school records for the period 1979-84. School absences, previously shown to reflect incidence of predominantly infectious diseases, were found to be positively correlated with occurrences of SIDS. The role of ordinarily nonlethal infections in occurrences of SIDS has been established by history, histology, and viral isolation. The authors concluded that the seasonality of SIDS is related to the seasonality of respiratory infections and that the seasonality is influenced by respiratory infections being spread from school children to infants during periods when schools are in session.