In a prospective longitudinal study with death as the end point in 17 middle-aged patients with Down's syndrome, dementia was clinically diagnosed in 15 patients, by means of careful observations in daily circumstances. Autopsies were performed in 10 cases: 8 demented patients and 2 nondemented patients. Neuropathologically, Alzheimer-type abnormalities were demonstrated in 9 patients, both demented and nondemented, and combined Alzheimer-type abnormalities with infarctions were demonstrated in 1 patient. In the 14 demented patients who did not show evidence of cerebrovascular or systemic vascular disease, dementia had an early onset and was rapidly progressive (mean age at onset, 51.3 years in the moderately retarded patients and 52.6 years in the severely retarded patients; mean duration of symptoms, respectively, 4.9 and 5.2 years). Cognitive and behavioral decline corresponded to symptoms of dementia of the Alzheimer's type in patients without Down's syndrome, but often were not recognized early. In the present group of patients, there was a remarkably high incidence of gait and speech deterioration. Also, the incidence of epileptic seizures and myoclonus was about eightfold, as compared with dementia of the Alzheimer's type in patients without Down's syndrome.