Abstract Erythrocytes from psoriatic patients have a significant increase in polyunsaturated fatty acids ( p < 0.001) especially in arachidonic acid ( p < 0.001). Glutathione peroxidase activity, in both erythrocytes and platelets, was stimulated when compared with normal cells ( p < 0.001, < 0.02, respectively) and the production of malondialdehyde was also increased in psoriasis ( p < 0.01). The level of plasma selenium was significantly reduced (52.80 vs 72.49 ng/ml; p < 0.001). α-Tocopherol and retinol were both normal in plasma of psoriatics. After two months of fish oil supplementation, the erythrocyte lipid pattern was changed, eicosapentaenoic and dochesaenoic acids substituting the arachidonate in the membrane. A reduction in malondialdehyde ( p < 0.01), a prolongation of bleeding time ( p < 0.05) and a further stimulation of glutathione-peroxidase ( p < 0.001) in both erythrocytes and platelets was also found.