The pneumococcal polysaccharide vaccine is recommended as a means of preventing invasive disease in the elderly. We compared responses to the 23-valent polysaccharide vaccine in 46 previously unvaccinated, healthy, institutionalized elderly persons (mean age, 85.5 years) with those in 12 healthy younger adults (mean age, 37 years) by measuring prevaccination and postvaccination serum IgG antibody concentrations (by ELISA), functional antibody activity (by opsonophagocytosis), IgG antibody avidity, and passive protection in mice. Postvaccination IgG antibody concentrations for two serotypes (6B and 19F) of the five studied (4, 6B, 14, 19F, and 23F) were significantly lower in elderly than in younger adults; however, opsonophagocytic activity was significantly reduced for all serotypes in the elderly. Sera with reduced opsonophagocytic activity (titer, <64) correlated with low IgG antibody avidity and protected mice poorly against pneumococcal challenge. In elderly persons receiving polysaccharide vaccination, there was a significant reduction in the functionality of postvaccination antibodies, and this appeared to increase with advanced age.