A total of 272 biopsies (from psoriatic lesions with a diameter of 2-10 cm) were performed on 178 psoriasis patients. Sections from these biopsies were stained with haematoxylin-eosin and by the PAS and trichrome techniques. Each section was investigated for the presence or absence of para- and hyperkeratosis, acanthosis, mitoses, intercellular edema and spongiosis of the epithelial ridges, edema at the top of the connective tissue-papillae, the number of leucocytes penetrating the epidermis at the top of these papillae, classical Munro abscesses, abscesses in the horny layer and Kogoj spongiform pustules, and the results were noted. It was found that the site of the biopsy in the psoriasis lesion, the location of the psoriasis lesion from which material for biopsy was taken and the moment of time during the course of the disease at which the biopsy was performed, had no influence on the histological signs observed in a given patient. In other words, the histological picture of psoriasis plaques from 2-10 cm found in a given patient exhibits a high degree of constancy. Some evidence was found indicating that signs of latent psoriasis may probably be present in the healthy skin of a psoriasis patient. It is generally assumed that the epidermis above the connective tissue-papillae in cases of psoriasis is thinner than normal. Our investigation showed that the reverse is the case: the thickness of the epidermis in the psoriasis efflorescences is significantly greater than in healthy skin nearby in the same patient.