A decrease in cognitive functions up to the development of dementia in the elderly is associated with a decrease in the blood level of polyunsaturated fatty acids, especially Omega-3, which occurs against the background of oxidative stress. The paper presents a comparative analysis of the spectrum of polyunsaturated fatty acids and the activity of individual components of the enzymatic antioxidant system in the blood of elderly people with impaired cognitive performance to the level of «mild cognitive decline» (MCI AD, prodromal Alzheimer's disease) or vascular etiology (MCI VaD, prodromal vascular dementia) compared with older people without signs of cognitive impairment. A decrease in the concentration of Omega-3 polyunsaturated fatty acids in the blood of both groups of the examined patients was revealed compared with the control group. In patients with AD MCI, a sharp decrease in the concentration of arachidonic acid (Omega-6) was detected compared with patients with MCI VaD and the control group. The decrease in the activity of the antioxidant enzymatic system and the decrease in polyunsaturated fatty acids due to their peroxidation revealed in this study indicate an intensification of the OS processes in patients with impaired cognitive functions. The question of the pathogenetic role of arachidonic acid in patients predisposed in the future to the development of AD is discussed.