The results reported in connection with previously published data suggest the importance of increased lipid peroxidation in the epidermis after Dithranol treatment. Malonaldehyde, one of the main products of lipid peroxidation, reacts with epidermal phosphofructokinase and glucose-6-phosphate dehydrogenase to cause enzyme inactivation by cross-linking of the enzyme protein demonstrated by dodecyl sulphate electrophoresis. Enzyme inactivation leads to an inhibition of hexose monophosphate shunt and glycolysis. The effect on energy donating metabolism in psoriatic lesions is discussed.