Reactive oxygen species (ROS) are essential for proper functioning of the body. It is important to strike a balance between the production of ROS and their removal by the action of the antioxidant system. The deterioration of this balance may lead to oxidative stress, which causes damage to cell components of all – of lipids, proteins and DNA. Lipid peroxidation is a chain and free-radical process of lipid oxidation, in the course of which are involved ROS. The lipid peroxidation may play an important role in the pathogenesis of psoriasis. The aim of the study was to determine the concentration of thiobarbituric acid reactive substances, malondialdehyde and conjugated dienes, and the activity of superoxide dismutase in patients with psoriasis. Material and methods: The study group consisted of 12 women and 24 men with psoriasis – patients of the Department of Dermatology, Sexually Transmitted Infections and Immunodermatology CM UMK. The control group consisted of 16 healthy volunteers. The material for the study was venous blood. The concentration of TBARS and CD was determined in plasma and erythrocytes. The concentration of MDA was determined in plasma and SOD activity in the erythrocytes. Results: Statistically significantly higher concentration of MDA and statistically significantly higher concentration of TBARS was revaled in patients with psoriasis as compared to control group. There were no statistically significant differences in plasma concentrations of CD among patients with psoriasis and control group. In erythrocytes of patients concentration of CD was twice as high. There were no statistically significant differences in the activity of SOD between the patient group and the control group. Conclusions: The most important product of lipid peroxidation responsible for the formation of psoriatic lesions is MDA. Lack of changes in the concentration of the primary products of lipid peroxidation, with significant changes in the concentration of secondary products of the process in blood of studied subjects can provide a significant degree of severity of damage to the cells.