Since May 2009, the pandemic influenza A(H1N1) virus has been spreading throughout the world. Epidemiological data indicate that the elderly are underrepresented among the ill individuals. Approximately 1,000 serum specimens collected in Finland in 2004 and 2005 from individuals born between 1909 and 2005, were analysed by haemagglutination-inhibition test for the presence of antibodies against the 2009 pandemic influenza A(H1N1) and recently circulating seasonal influenza A viruses. Ninety-six per cent of individuals born between 1909 and 1919 had antibodies against the 2009 pandemic influenza virus, while in age groups born between 1920 and 1944, the prevalence varied from 77% to 14%. Most individuals born after 1944 lacked antibodies to the pandemic virus. In sequence comparisons the haemagglutinin (HA) gene of the 2009 pandemic influenza A(H1N1) virus was closely related to that of the Spanish influenza and 1976 swine influenza viruses. Based on the three-dimensional structure of the HA molecule, the antigenic epitopes of the pandemic virus HA are more closely related to those of the Spanish influenza HA than to those of recent seasonal influenza A(H1N1) viruses. Among the elderly, cross-reactive antibodies against the 2009 pandemic influenza virus, which likely originate from infections caused by the Spanish influenza virus and its immediate descendants, may provide protective immunity against the present pandemic virus.