This population study describes the experience with primary intracerebral hemorrhage (PIH) in residents of Rochester, MN, for the 32-year period from 1945 through 1976. The average annual age-adjusted incidence rate for PIH was 12.1 per 100,000 population, and the incidence for all cases of spontaneous intracerebral hemorrhage was 15.2 per 100,000. The PIH rates were higher for males than for females, and they increased steadily with age. When patients on long-term anticoagulant therapy were excluded, there was a steady decrease in the average annual age-adjusted incidence rate for PIH in each succeeding 8-year interval since 1945. Prehemorrhage hypertension, present overall in 89% of patients, was much more frequent and severe in the earlier years of the study. The frequency and severity of prehemorrhage hypertension also varied inversely with age in the population with PIH. The median age at the onset of PIH increased from 65 years for the period 1945 through 1952 to 71 years for 1969 through 1976.