The blood lipid composition (plasma, platelets and leukocytes), platelet membrane fluidity, apolipoproteins A and B in the plasma of AD patients and control subjects with distinct Apo E genotypes were investigated. No significant differences were found between the Apo E genotype and the cholesterol, phospholipids, triglycerides and Apo B levels in the plasma; cholesterol and phospholipids levels in platelet and leukocyte membranes; and platelet membrane fluidity of AD and control groups. However, the phospholipid levels in the leukocyte membranes of the control subgroup with the genotypes epsilon3/epsilon3 and epsilon3/epsilon4 and the AD subgroups with the genotypes epsilon2/epsilon3 and epsilon3/epsilon3, epsilon3/epsilon4 and epsilon4/epsilon4 were significantly lower than those observed in the control subgroup with the genotype epsilon2/epsilon3. Moreover, the cholesterol and phospholipid levels in the platelet membranes of the AD subgroup with the epsilon2 allele were significantly higher than those in the AD subgroup without the epsilon2 allele and the control subgroups with and without the epsilon2 allele. A strong correlation was found between cholesterol and phospholipids levels in the platelet membranes of the AD and control subgroups without the epsilon2 allele, but the residual cholesterol level in the platelet membranes of the AD subgroup was twice that observed in the control subgroup. Furthermore, the Apo A levels in the plasma of the AD subgroup with the epsilon3 allele were significantly lower than those observed in the AD subgroup without the epsilon3 allele and the control subgroup with the epsilon3 allele. The results are discussed in terms of involvement of lipid metabolism in the etiopathogenesis of AD.