Clinicopathologic features of Alzheimer disease, the commonest cause of presenile or senile dementia, are presented. Several of the microscopic brain lesions found in patients with this dementia share the staining properties of amyloid and at least two of these lesions (senile plaque cores and amyloid angiopathy) are biochemically identical. Theories pertinent to the origins of brain amyloid and its role in the pathogenesis of Alzheimer disease are discussed in relation to theories of the cause of this dementia. Possible treatments for Alzheimer disease developed from our knowledge of brain amyloid processing and biochemistry are considered.