Using the centralized diagnostic data resource for the population of Rochester, Minnesota, we determined the prevalence of medically diagnosed dementia for this community. There were 289 individuals with dementia who were residing in Rochester on January 1, 1975: 208 women and 81 men. The overall age- and sex-adjusted rates per 100,000 population were 342 for all dementias and 227 for Alzheimer's disease. Prevalence rates increased with age for both sexes. Limiting the denominator to persons greater than or equal to 65 years, age-adjusted rates per 100,000 population for men and women were 3,452 and 3,466 (or about 3.5%), respectively, for dementia and 2,007 and 2,675 (2.0 and 2.7%), respectively, for Alzheimer's disease.