Sections from 100 psoriatic lesions commonly showed parakeratosis, Munro microabscesses, a diminished or absent granular layer, acanthosis, papillomatosis, tortuosity and dilatation of capillaries, edema, and chronic inflammation in the upper corium. Vacuolization, disruption and hydropic degeneration of the basal cells above the tips of the dermal papillae associated with exocytosis of polymorphonuclear leukocytes and spongiosis were found to be important features which have not been generally emphasized. Clinically normal skin of psoriatic patients showed histologic and enzymatic evidence of altered metabolism. Metabolism in the pentose-monophosphate shunt, as evidenced by glucose-6-phosphate dehydrogenase activity, was markedly increased in the keratinizing zone in the psoriatic lesion. This change was also found in certain other dermatoses and is not specific for psoriasis.