Trichoepitheliomas and many basal cell carcinomas appear to arise from the hair follicle, and in particular from the hair follicle bulge. This histogencsis is suggested from both morphological and immunohisto-chemical studies on tumor cells and siroma. Epithelial stem cells are thought to be important in tumorigenesis, and we previously localized a population of stem ceils to the bulge area of the outer root sheath. We recently identified an anti-CD8 monoclonal antibody (DAKO clone C8/144B) that cross-reacts with cytokeratin 15 (K15), and serves as a specific marker for the bulge. In this study, we screened a Series of trichocpitheliomas (n=13), basal cell carcinomas (n=37) and a variety of other skin tumors with this antibody, All trichocpitheliomas (100%) showed keratin 15 expression, while only a subset of basal cell carcinomas (27%) was K15-positive. Epidermal tumors, including squamous cell carcinomas, were K15-negative. Tumors of follicular derivation such as proliferating trichilemmal cysts were also K15-positive, while others such as pilomatricoma were K15-negative. Expression of K15 in trichoepitheliomas, some basal cell carcinomas and other follicular tumors suggests that these tumors are related to hair follicle stem cells in the bulge.