Annual vaccination against influenza virus is generally recommended to elderly and chronically ill, but the relative importance of factors influencing the outcome is not fully understood. In this study of 88 individuals all aged 69 years, the increase in haemagglutinin-inhibiting (HI) antibodies to trivalent inactivated influenza vaccine was correlated with HI titres before vaccination, prior vaccinations against influenza, cytomegalovirus serostatus and, as an estimate of immune risk profile, the ratio between CD4 + and CD8 + T cells. Vaccine responses were impaired by high pre-existing HI antibody titres. For influenza B repeated vaccinations and an inverse CD4/CD8 ratio had a negative impact on the vaccine response. Cytomegalovirus seropositivity had no apparent effect on HI titres before or after vaccination. It is concluded that both pre-existing HI antibodies and previous vaccinations to influenza may influence the humoral response to influenza vaccination and that a CD4/CD8 ratio < 1 may indicate an impaired ability to respond to repeated antigenic stimulation.