Oxidative stress is believed to play a major role in the pathogenesis of Alzheimer disease (AD). Plasma concentrations of vitamins C, A and E, of uric acid, thiols and carotenoids were lower and of malondialdehyde (MDA) higher in 35 AD patients (85.9 +/- 5.5 y) compared to 40 controls 85.5 +/- 4.4 y). Differences were significant for vitamin C, vitamin E, lutein, lycopene, alpha-carotene and MDA (p < 0.001). Plasma exposed to peroxyl radicals showed a rate of antioxidant consumption and of MDA production higher in AD patients than in controls. AD in advanced age is accompanied by a poor plasma antioxidant status and increased plasma lipid peroxidation, as well as by a low resistance to peroxyl radical exposure.