An epidemiological study of dementia was carried out in the city of Turku (population 164,568) in Finland. A total of 421 patients with moderate to severe dementia were found. Degenerative dementia, i.e. presenile and senile dementia, was present in 218 patients (51.8%), and in 152 patients (36.1%) the dementia was associated with arteriosclerosis, including multi-infarct and combined dementia. The prevalence rate of dementia, all types, was 256 per 100,000 population, and 1,961 per 100,000 population over 65 years old. The age-specific prevalence rates of dementia increased with advancing age from 0.1% in the age group 55-64 years to 11% in the age group over 85 years. The peak annual incidence rate for all types of dementia was 58 per 100,000 population, and 447 per 100,000 population over 65 years old. Both prevalence and incidence figures suggested a female preponderance in dementia of degenerative origin and in dementia with associated arteriosclerosis.