Substantial evidence supports the hypothesis that oxygen free radicals are involved in various neurodegenerative disorders. To assess the presence of oxidative stress in Alzheimer's disease (AD) we examined the activity of the enzyme copper-zinc superoxide dismutase (CuZnSOD) in red blood cells, the levels of the mitochondrial inducible enzyme manganese superoxide dismutase (MnSOD) mRNA in lymphocytes, and the total radical-trapping antioxidant capacity (TRAP) in plasma of AD patients and in a group of age-matched non-demented controls. We found that CuZnSOD activity (P < 0.01 vs. controls) was significantly increased as well as the MnSOD mRNA levels while the total antioxidant status (P < 0.001 vs. controls) was decreased in AD patients. These findings support the role of oxidative alterations in the pathogenetic mechanism underlying AD neurodegeneration.