Paraoxonase activity, homocysteine level and lipids were determined in 120 patients with dementia (51 with Alzheimer disease, 28 with dementia of vascular origin, 41 with mixed dementia), 45 with mild cognitive impairment and in 61 age and sex matched controls without dementia. Paraoxonase activity was decreased in Alzheimer disease and in mixed dementia as compared with control group. In the same forms of dementia homocysteine levels were increased. In Alzheimer disease paraoxonase activity was negatively correlated with homocysteine levels. Minimental State Examination results showed positive correlation with paraoxonase activity. The results suggest an important role of oxidative stress in the development of the forms of dementia with prevailing neurodegeneration.