In the epidermal cells of patients suffering from psoriasis we found a significant prolongation of DNA-synthesis time (ts) in uninvolved skin, very early lesions, and fully developed plaques. In uninvolved psoriatic skin ts in addition increased significantly within 6 hours after stripping of the horny layer. In normal epidermis and in other states of epidermal inflammation and hyperproliferation (akanthosis by petrolatum, toxic dermatitis, chronic allergic ekzema, neurodermitis, allergic patch test reaction) a comparable prolongation of ts was not ascertainable. This prolongation is most distinct in the early lesions and proceeds the development of hyperproliferation and akanthosis. A dermal infiltrate with increased proliferative activity seems to be a stimulus, in the sense of a Koebner-phenomenon. The abnormal psoriatic epidermis, with disturbed DNA-synthesis, reacts to this infiltrate as well as to other irritants not with a limited hyperproliferation but with the development of psoriatic plaque.