The frequencies of anencephalus and spina bifida in births reported from the Rotunda Hospital, Dublin, Ireland, were examined and compared with trends observed in deliveries in the Boston Lying-In and Providence Lying-In Hospitals, U.S.A., for the period 1900–1965. In these quinquennial data two epidemics of anencephalus which peaked during the years 1935–39 and 1960–65 were noted in Dublin. The former resembled the epidemic of neurological malformations in the north-east United States which reached a maximum during the period 1930–34. A striking difference in trends in the incidence of these defects was evident over the years 1940–65, the frequency markedly declining in this area of the U.S.A. whilst increasing in Dublin. Associations of these long-term secular variations in incidence are discussed in relation to the influenza pandemic 1918–19 and changes in the production and consumption of alcohol in Ireland and the U.S.A. during this century.