The keratinocytes in human epidermis are replaced by a population of stem cells located in the basal layer of the epidermis and one candidate stem cell marker is the transcription factor p63. We studied the expression of p63, immunohistochemically, in various hyperproliferative skin diseases (10 poorly differentiated metastatis squamous cell carcinomas (SCCs), 10 non-metastatic primary cutaneous SCCs, 10 cases of Bowen's diseases, 10 actinic keratosis, and 10 melanomas) and also observed the change of p63 expression in psoriasis after cyclosporine treatment. p63 was normally expressed in basal layer cells. Poorly-differentiated metastatic SCC showed the highest expression in most of the tumor cells, while psoriasis, actinic keratosis, Bowen's disease and primary SCC showed an increased expression in the basal and suprabasal area compared to in normal epidermis. The cyclosporine treatment in psoriasis reduced the expression of p63 to a normal level. This data suggests that p63 expression may influence tumor cell differentiation and proliferation without a direct tumorigenesis effect in epithelial tissue.