Increased oxidative stress may accompany disease of the central nervous system including dementia. The brain is especially susceptible to damage mediated by reactive oxygen species because it has a high rate of oxygen consumption and contains large amounts of readily oxidisable substrates, such as polyunsaturated fatty acids. Indices of systemic oxidative stress, including serum lipid peroxidation may be greater in dementia than in normal ageing. Study groups consisted of 30 patients with dementia and 18 healthy age-matched controls. Ali patients underwent neuropsychological testing and qualified for the study on the basis of history, physical examination, complementary laboratory tests, and brain computed tomography scan. Serum levels were assessed for the following lipid peroxidation products: conjugated dienes, lipid peroxides, thiobarbituric acid reactive substances, and Schiff bases. There were two statistically significant differences in serum levels of lipid peroxidation product between the study groups. Lipid peroxides were significantly lower (0,34±0,03 absorbance units/ml versus l, 12±0,96, p=0,0055), while Schiff bases were significantly higher in the subjects with dementia (589,4±267,3 arbitrary fluorescence units/ml versus 329,0±107,5 in healthy, p=0,000282). Cognitive impairment did not correlate with levels of lipid peroxidation products.