Light and electron microscopic features in the presenile dementia that occurred in a 54 year old woman with Down's syndrome were identical to those of Alzheimer's presenile dementia. In both diseases, senile plaques and Alzheimer's neurofibrillary tangles fill the cerebral cortex and are scattered through subcortical and brainstem nuclei. Although presenile dementia of Down's syndrome is ill-defined clinically, the marked histologic changes are so predictably age-related that this condition offers many insights into sequences of aging and degeneration in the human brain.