Recent evidence suggests that a polymorphism in the regulatory region of the apolipoprotein E gene (APOE) is associated with an increased risk for developing Alzheimer's disease (AD) independent of that conveyed by the epsilon4 allele of APOE. Previous work by our group indicated that plasma apolipoprotein E (apoE) levels were elevated in AD, raising the possibility that the -491 genotype might modify AD risk by increasing expression of the APOE gene. In a total of 638 individuals the -491AA genotype was significantly associated with AD (P < 0.005) while the TT genotype was associated with controls (P < 0.005). In 138 individuals the AA genotype showed significantly higher plasma apoE levels, independent of epsilon4 and AD status (P < 0.01) as well as within control and AD groups (P < 0.05). Within the AD group the AA genotype showed increased apoE levels when compared to AA controls (P < 0.0001). These results suggest that the -491 AA genotype is associated with increased plasma apoE levels, providing a potential basis for elucidating how that genotype increases the risk for developing AD.