Alzheimer's disease is the leading cause of dementia in the elderly. Most patients do not have an obvious family history and are classified as sporadic. Genetic factors in early and late-onset Alzheimer's disease are now well documented. This paper reviews the known genetic loci associated with Alzheimer's disease with an emphasis on the role of apolipoprotein E in late-onset familial and sporadic disease. Apolipoprotein E allele 4 is a susceptibility gene that is found in approximately 50% of patients with Alzheimer's disease. Hypotheses about apolipoprotein E function that may lead to Alzheimer's disease are also discussed.