This study characterizes possible confounders that might make cohorts vaccinated and unvaccinated against influenza and pneumococcal infection different at baseline, with the hypothesis that the two cohorts are comparable. The similarity between health and demographic data was analysed by a randomized, multivariant study addressed to 10 000 persons aged 65 years and older in Stockholm County and was carried out in the form of a postal inquiry during the period December 2000 to May 2001. The study-population response rate was 78%. Of these, 66% (5120 persons) had been given at least one influenza vaccination during the 3-year study period (1998–2000), 50% (3780) had received one pneumococcal vaccination and 78% had received both vaccines during the period. The vaccination rate was lower in the age group 65–69 years (60%), compared with elderly cohorts aged over 70 years (67–72%, P<0·001). Elderly persons living in nursing homes or institutions had higher vaccination rates than persons living in their own households (72 vs. 67%). Persons with underlying chronic diseases had higher vaccination rates (71%, P<0·001) than those without underlying chronic diseases. Vaccine recipients had fewer days in hospital, compared with non-recipients. Unvaccinated persons with myocardial disease had nine times more days in hospital than vaccinated persons with myocardial disease. Vaccination against pneumococcal infection had an additional effect with influenza vaccination in reducing hospitalization for chronic lung diseases; influenza vaccination alone did not have this effect. In conclusion, the influenza and pneumococcal-vaccine recipients were older and had significantly more chronic lung and heart conditions than the unvaccinated cohort.