Patients from five generations of a family developed presenile Alzheimer's disease (AD) early in the fifth decade. Recent memory, information-processing speed, sequential tracking, and conceptual reasoning were the earliest cognitive functions affected. Language and visuoperceptual skills were largely spared early in the course of the disease. Later, there were progressive cognitive deficits and inability to perform the activities of daily living. Death occurred, on average, 6 years after onset. Three autopsies in affected members revealed cerebral amyloid deposits and neurofibrillary tangles. Clinical and pathologic features were typical for familial AD. Direct DNA sequencing revealed a G→T change at position 2149 of the amyloid precursor protein (APP) gene that resulted in the substitution of phenylalanine for valine in the transmembrane domain of the mature protein. This mutation was present in DNA from all four examined affected individuals and linked to the disease with a lod score of 3.25, and was the most probable cause of AD in this family.