--The authors study in psoriasis an enzyme of the glucolytic way, the Phospho-hexose-isomerase. --They find significant high levels in blood and damaged skin, while these were "normal" in healthy skin. --Phospho-hexose-isomerase acts as a biological marker in psoriasis, and acquires the significance of an active cellular proliferation. --The highest values are those of the erythrodermaes which become normal with clinical involution. --The authors suggest that psoriasis, by an exaggerated cellular division inhibition mechanism, metabolism is deviated in order to bring the energetic elements and substrate to form the new cell. --Phospho-hexose-isomerase is not specific of the malignant process.