The apolipoprotein E locus (APOE) is associated with variations in the age of onset and risk of Alzheimer's disease. The APOE4 allele increases the probability of disease at an earlier age. In contrast, the APOE3 and APOE2 alleles decrease the probability of disease and increase the age of onset. Therefore the metabolism of apolipoprotein E is relevant to Alzheimer's disease. Isoform-specific interactions of apolipoprotein E with other molecules may determine the rate of disease expression through molecular pathways that appear unique to the disease. In addition, some isoform-specific interactions of apolipoprotein E have been demonstrated with the defining pathological lesions of Alzheimer's disease, the neurofibrillary tangle and neuritic plaque. Several hypotheses of disease pathogenesis are now based on the relevance of apolipoprotein E.