Alzheimer disease will affect increasing numbers of people as baby boomers (persons born between 1946 and 1964) age. This work reports projections of the incidence of Alzheimer disease(AD) that will occur among older Americans in the future. Education adjusted age-specific incidence rates of clinically diagnosed probable AD were obtained from stratified random samples of residents 65 years of age and older in a geographically defined community. These rates were applied to U.S. Census Bureau projections of the total U.S. population by age and sex to estimate the number of people newly affected each year. The annual number of incident cases is expected to more than double by the midpoint of the twenty-first century: from 377,000 (95% confidence interval = 159,000-595,000) in 1995 to 959,000 (95% confidence interval = 140,000-1,778,000) in 2050. The proportion of new onset casess who are age 85 or older will increase from 40% in 1995 to 62% in 2050 when the youngest of the baby boomers will attain that age. Without progress in preventing or delaying onset of Alzheimer disease, both the number of people with Alzheimer disease and the proportion of the total population affected will increase substantially.