Elderly individuals are at high risk for morbidity and mortality when infected with influenza virus. Vaccinations with inactivated virus are less effective in the elderly due to the declining competency of the aging immune system. We have explored whether immunological parameters predict poor anti-influenza virus vaccine responses and can be used as biological markers of immunosenescence. One hundred fifty-three residents of community-based retirement facilities aged 65 to 98 years received a trivalent influenza vaccine. Vaccine-induced antibody responses were determined by comparing hemagglutination inhibition titers before and 28 days after immunization. The composition of the T-cell compartment was analyzed by flow cytometry and the sizes of three T-cell subsets, CD4⁺CD45RO⁺ cells, CD4⁺ CD28^null cells, and CD8⁺ CD28^null cells, were determined. Only 17% of the vaccine recipients were able to generate an increase in titers of antibody to all three vaccine components, and 46% of the immunized individuals failed to respond to any of the three hemagglutinins. The likelihood of successful vaccination declined with age and was independently correlated with the expansion of a particular T-cell subset, CD8⁺ CD28^null T cells. The sizes of the CD4⁺ CD45RO⁺ memory T-cell and CD4⁺ CD28^null T-cell subsets had no effect on the ability to mount anti-influenza virus antibody responses. Frequencies of CD8⁺ CD28^null T cells are useful biological markers of compromised immunocompetence, identifying individuals at risk for insufficient antibody responses.