Neuropathologic and neurochemical studies of older adults with Down's syndrome and those with Alzheimer's disease reveal striking similarities. Genetic studies indicate that near relatives of patients with Alzheimer's disease are at increased risk of developing Alzheimer's disease, and the risk appears to be age specific. These families with familial Alzheimer's disease have also been found to have a high incidence of Down's syndrome. Neurochemical data suggest that a cholinergic deficiency must be present for dementia to develop, and serial assessments of brain metabolic function with positron emission tomography in Alzheimer's disease have shown that the parietal lobe has reductions in metabolic function before the onset of neuropsychologic deficits in this brain region. Neuropsychologic testing indicates that patients with Down's syndrome over 35 years old have poorer cognitive skills than do younger patients. Brain metabolic function is excessively reduced in the demented adults with Down's syndrome.