Cation transport systems and lipid composition of erythrocyte membrane were studied in 27 psoriatic patients and in 34 healthy individuals. Whereas intracellular Na and K content, Na- and K-passive permeability and Li-Na countertransport of psoriatics did not show any statistical difference from normals, the Na/K ATPase pump activity was significantly higher and Na-K cotransport was significantly lower. Membrane lipid composition of psoriatics was different from normals: an increase in arachidonic acid and in unsaturated (poly- and total unsaturated) fatty acid content was found. A positive correlation was demonstrated between unsaturated/saturated fatty acid ratio and Na/K ATPase pump activity. These results demonstrate an alteration of erythrocyte Na/K ATPase pump and Na-K cotransport in psoriasis. These alterations of cation transport are associated with a perturbation of membrane fatty acid composition which appears a widespread phenomenon in cells of psoriatic patients.