BACKGROUND Experimental approaches using transplantable human tumors in nude mice have greatly improved studies on their pathogenesis and treatment. OBJECTIVE We studied the establishment of a transplantable tumor, EMP-K1, derived from extramammary Paget's disease and the morphology and unique hormone dependence of this tumor. METHODS Tissue fragments from a metastatic skin tumor of an 80-year-old man with primary genital extramammary Paget's disease were implanted into male nude BALB/c mice. Tissue fragments of the established tumor were implanted into 50 castrated male 8-week-old nude mice, which were given injections of 100 micrograms testosterone propionate (TP), 100 micrograms 5 alpha-dihydrotestosterone (DHT), 10 micrograms diethylstilbestrol (DES), or 1, 10, or 100 micrograms 17 beta-estradiol (E2). All injections were administered intramuscularly once daily, starting from the day of transplantation. The established tumors were examined immunohistochemically and biochemically. RESULTS A transplantable tumor (EMP-K1) was established in the nude mouse. EMP-K1 tumor cells expressed antigens such as carcinoembryonic antigen and epithelial membrane antigen, cytokeratin, and c-erbB-2 protein and contained androgen, estrogen, and progesterone receptors. The growth of the EMP-K1 tumor was stimulated by TP, DHT, DES, and E2. CONCLUSIONS These results suggest that both androgen and estrogen may stimulate the growth of the same tumor by both androgen receptor and estrogen receptor pathways. CONCLUSIONS The EMP-K1 tumor is a useful tool for studies on the biology of extramammary Paget's disease, and further studies using these tumors will provide useful information concerning proper hormone therapy.