Although awareness of Alzheimer's disease has increased strikingly among professional and lay audiences, precise means do not exist to calculate how many Americans the disease affects. Projecting data from a cohort of 32,000 individuals in a defined community in the United States, however, suggests that there were 2.88 million persons aged 65 or older with probable Alzheimer's disease in 1980. Using U.S. Census Bureau middle series projections yields an estimated total of 10.3 million persons with the disease for the year 2050. Limitations on these estimates notwithstanding, the projections indicate that more studies are needed to identify the etiology of Alzheimer's disease, its incidence, and the future costs of relevant institutional and community care.