In this study, for the first time to our knowledge, all new cases of Alzheimer's disease and other dementing illnesses have been documented in a well-defined United States community. A total of 178 patients in an average annual at-risk population of 18,991 persons over age 29 developed dementia during 1960 through 1964 in Rochester, Minnesota, yielding an average annual incidence rate of 187.5 new cases/100,000/year. The corresponding rate for clinically and/or pathologically diagnosed Alzheimer's disease (the most common cause of dementia in this community) was 123.3 new cases/100,000/year, based on 117 cases. The incidence rates for both dementia in general and Alzheimer's disease in particular rise dramatically with age. All patients were followed to death or to 1982, and the median survival was 63 months. These data include only those in the community who came to medical attention, but they yield higher rates than studies in Scandinavia, indicating a relatively high level of case ascertainment.