In order to gain insight into metabolic functions, it is the task of histochemistry to show the presence of chemical substances in cells and tissues with the use of sensitive methods in histological sections.88 If one considers the histochemical findings in psoriasis in this light, there is no doubt that many of them can be related to biochemical findings that in recent times have led Rothman,'". lz7 Griineberg and S~a k a l l ,~7 and Flesch and EsodaJ5 to newer concepts concerning the keratinization process and the nature of psoriasis.The histological picture of psoriasis is dominated by a marked acanthosis and parakeratosis with the formation of Munro's abscesses in the epidermis while, in the dermis, a n inflammatory process is present, especially in the papillary portions. When discussing the histochemistry of psoriasis, one could use these features as guideposts in an attempt to clarify the nature of the disease. This approach has been followed by Steigleder in his extensiye study of the histochemistry of a~a n t h o s i s~~~ and by B r a~n -F a l c o ,~~ Spier and Caneghem,I4O and Steigle~ler'~9 in their detailed works on parakeratosis. However, for the purpose of the present review, it seems to be more appropriate to divide the material on the basis of the histochemical localization of various chemical compounds within the epidermis and dermis with a brief discussion of their functional significance; our knowledge in this last area is still very scant.The following topics will be reviewed: (1) the histochemistry of the epidermis in psoriasis, including the histochemical localization of inorganic substances; of carbohydrates; of lipids; of proteins, amino acids, sulfhydryl groups, and nucleic acids; and of enzymes; and (2) the histochemistry of the corium in psoriasis, including that of the basement membrane; of the capillaries; and of the ground substance.