A major decline in the incidence of stroke occurred in the population of Rochester, Minnesota, during the period 1945 to 1974. For every 100 first episodes of stroke that occurred per unit of population during the period 1945--49, only 55 occurred in the period 1970--74. Although the decline was present in both sexes and in all age groups, the reduction in rates was more pronounced in the elderly. There was no major change in age at onset. Analysis of cohorts born during successive five-year periods from 1865 to 1915 confirmed the decreasing incidence rate in all age groups.