Institutionalized patients with Down syndrome (N=307) were monitored for 5 to 10 years to determine prevalence of Alzheimer-type dementia. Prevalence increased from 11% between ages 40 and 49 to 77% between 60 and 69. All patients 70 and over had dementia. Mean age of onset of dementia was 56 years. Neuropathological findings were consistent with clinical diagnosis. (Author/DB)