Recent improvements in medical care have markedly increased the life span of individuals with Down syndrome, and many older people with Down syndrome exhibit changes in health and functional capacities suggestive of premature or accelerated aging. Indeed, there is strong evidence that there is in this population a unique susceptibility to Alzheimer's disease. However, existing studies of the association between Down syndrome and Alzheimer's disease have been limited with respect to their design, sample size, and subject selection criteria. The present investigation will collect longitudinal data regarding age-related changes in adaptive behavior in a large group of middle aged to older people with Down syndrome, and compare them to a matched group without Down syndrome. Matching variables will be age, sex, residential placement, and severity of mental retardation. Adaptive behavior will be assessed using the A.A.M.D. Adaptive Behavior Scale, a standardized and reliable instrument. The largest and most representative sample of older people with Down syndrome will be studied, and the most accurate data on the aging in mentally retarded individuals collected to date will be provided. If, as we expect, people with Down syndrome do regress in functioning as they age, results of this study will be used to describe the progression of symptoms. Additionally, because a high rate of autopsy among study participants who die is anticipated, a determination of the relationship between the extent of regression in adaptive behavior and neuropathological changes diagnostic of Alzheimer's disease will be made.