A study of the incidence of diagnosed bacterial meningitis in Olmsted County, Minnesota showed that there was a threefold increase in bacterial meningitis in the elderly and in Haemophilus infiuenzae meningitis from the period 1935–1946 to the period 1959–1970. The proportion of fatal cases in patients 60 years of age and older increased from 10% during the years 1935–1946 to 58% during the years 1959–1970. This increase was related principally to the persistence of high case-fatality ratios in the elderly and decreasing case-fatality ratios in younger patients. In the period 1959–1970, three-quarters of all cases of bacterial meningitis were caused by H. infiuenzae, Diplococcus pneumoniae, or Neisseria meningitidis, but two-thirds of the deaths were caused by other bacteria.