Recently an association has been noted between a particular allele (epsilon4) of the APOE locus and Alzheimer's disease (AD) in samples of patients selected for study at an AD research center. Such samples are highly selected either for family studies or for clear diagnoses of AD. We wish to determine the allele frequencies at the APOE locus in an epidemiologically ascertained AD sample compared to controls from the same population. This will validate the findings of previous studies and will determine the true risk of AD conferred by alleles at this locus. We will also examine allele frequencies in other dementias from the same population. We will investigate the relationship between APOE genotypes and age of onset and progression in both the epidemiologic sample and a large cohort of AD families ascertained for genetic linkage studies.