The extension border of lesions of psoriasis vulgaris has been studied by histogeometric analysis of the epidermis and by autoradiography after in vitro incorporation of tritiated thymidine. The first modification observed is a hyperactivity of in the suprabasal layer associated DNA-synthesis with band-like acanthosis. Later, the basal labelling index is increased and the germinative compartment is exponentially enlarged by extension to more superficial layers, each of them reaching progressively the level of thymidine incorporation in the preceding one. These modifications could result from a defect of cellular differenciation perhaps through a decreased receptivity of the psoriatic epidermal cells to some inhibitory factors and a progressive selection of this population.