We report a case of a 74-year-old male with a trichilemmal tumor arising in a seborrheic keratosis on the buttock and the results of a cell kinetic study of this tumor using a BrdU staining method. The incidence of trichilemmal tumor arising in a seborrheic keratosis seems to be extremely rare. The labeling index of this tumor was 12.0%; this was a level intermediate between normal epidermis and a variety of hyperproliferative skin diseases such as squamous cell carcinoma, Bowen's disease, and psoriasis vulgaris. DNA replicating cells were present in the germinative layers in normal epidermis and the benign hyperproliferative skin diseases, psoriasis vulgaris. In contrast, DNA replicating cells were found throughout the entire epidermis in premalignant and malignant tumors such as in Bowen's disease and squamous cell carcinoma. In this case, DNA replicating cells were localized mainly in the basal and parabasal cell layers, but also seen in the upper squamous layers. These findings suggest that this trichilemmal tumor had a malignant tendency, though it was slow-growing and relatively benign in nature.