p63 is an oncogene belonging to the p53 gene family. In normal human skin, p63 is expressed by the least-differentiated keratinocytes of the epidermis and its appendages, by myoepithelial cells of sweat glands, and by a wide variety of primary tumors arising therefrom. The expression of p63 by metastatic skin tumors has been more controversial. In this study, we assessed the usefulness of p63 detection in the differential diagnosis between primary and secondary (metastatic) skin tumors. The expression of p63 was studied in 45 cases of cutaneous metastases, mostly of known primary origin, and 94 benign and malignant epithelial primary skin tumors. p63 was expressed by 85 (89%) of 96 primary skin tumors and 5 (11%) of 45 cutaneous metastases. These results suggest that p63 may be a useful adjunct in the diagnosis of skin carcinomas; however, contrary to what has been previously claimed, expression of p63 does not rule out the metastatic origin of a cutaneous carcinoma.