The micromorphological appearance of the psoriatic lesion is characterized by cutaneous inflammation, epidermal acanthosis and abnormal keratinization. The relevance of immunological factors has been reviewed in recent literature. Despite a longstanding interest of many centres in the pathogenesis of psoriasis, the position of epidermal proliferation, a key feature of the psoriatic plaque, remains to be established. The aim of the present review was to provide evidence from the literature and from our own work that enables us to answer the question to what extent epidermal proliferation might be a primary pathogenetic event or whether it represents a secondary phenomenon resulting from other abnormalities.