During a randomised controlled trial with the 23-valent pneumococcal vaccine in older persons, antibody concentrations and opsonophagocytic activity (OPA) were compared between eight patients who developed culture-verified pneumococcal pneumonia and 38 controls, matched for age, sex and vaccination status. Patients who developed pneumococcal pneumonia did not respond with a significant increase of antibody concentration (>1microg/ml) post-vaccination to the infecting serotype, but responded equally well as controls to most other serotypes. Neither was there any significant difference in the OPA post-vaccination between patients and controls. In conclusion, the 23-valent pneumococcal vaccine should be regarded as 23 different vaccines, rather than one. Older persons who fail to respond to one serotype may well be protected against infection by the other 22 serotypes.