This fact sheet summarizes the interrelations of Down syndrome and Alzheimer disease in a question and answer format. The following questions are addressed: What is Alzheimer disease? Why is a relationship suggested between Down syndrome and Alzheimer disease? Is there a genetic link between Alzheimer disease and Down syndrome? Why is a relationship between Alzheimer disease and Down syndrome doubted? Is dementia defined differently in persons with Alzheimer disease than in persons with Down syndrome? How can the research on the effects of Alzheimer disease in people with Down syndrome be summarized? What is the relation of aging to dementia in people with Down syndrome? What does the future hold for treatment and prevention? A source for further information is also given. (DB)