The histogenesis of extramammary Paget's disease has long remained unresolved and controversial. In an attempt to delineate the origin of the neoplastic cells in this disease, the immunoperoxidase localization of gross cystic disease fluid protein (GCDFP-15), a marker of apocrine epithelium, carcinoembryonic antigen (CEA), and keratin proteins, was determined for seven cases of extramammary Paget's disease (five vulvar, one anogenital, and one axillary). Immunoreactivity for GCDFP-15 was localized within Paget cells in six of our seven cases, including five cases from the vulva and one case from the axilla. CEA was present in the Paget cells in all seven cases. None of the Paget cells exhibited immunoreactivity for keratin proteins. Within normal skin, eccrine glands were immunoreactive for both keratin and CEA, whereas GCDFP-15 localized only to apocrine ducts and glands. Our findings strongly support an apocrine cell derivation for extramammary Paget's disease.