A patient with extramammary Paget's disease showed perianal nodules surrounded by erythema. rIFN-α-2a (300 million IU) was intralesionally injected three times a week. After the administration of 17, 700 million IU IFN-α, the skin lesions regressed leaving infiltrating plaques. Histologically, Paget cells were not observed at the site of erythema, but survived in the thickened epidermis. The intradermally infiltrating Paget cells remained. Then, the infiltrating lesions were excised under local anesthesia. Paget cells, however, were observed in the epidermis and in the dermis adjacent to the surgical suture. Paget cells completely disappeared and remission has been lasting for 6 months after electron beam irradiations (total 58 Gy). Intralesional IFN-α injections are effective on erythematous, superficial stage of extramammary Paget's disease, whereas it is very hard to get complete remission of advanced lesions by the intradermal IFN-α injections.