The morphological and biochemical characteristics of psoriasis are well documented, but the pathogenesis of this disease is not clearly understood. A variety of in vitro models of psoriasis have been developed in attempts to identify the trigger factors, but no model so far reproduces the stable psoriatic phenotype accurately. In the present work, we initially checked the immunohistochemical distribution of proliferation/differentiation markers in psoriatic skin in vivo, and our results largely confirm previously reported data. However the study was performed using a new series of monoclonal antibodies to keratin. Subsequently we took normal or psoriatic skin biopsies, reconstructed skin equivalents using a recently described model and analysed the proliferation/differentiation status of the resulting epidermis. Dramatic morphological and antigenic differences were found between normal and psoriatic skin in vivo, but whatever the source of the initial biopsy, a unique in vitro phenotype was obtained in the reconstructed epidermis. This phenotype was marked by mild hyperproliferation and an altered distribution of differentiation-associated antigens suggesting a need for extracutaneous stimuli to maintain the psoriatic phenotype in vitro.