Extramammary Paget's disease occurs commonly on the external female genitalia and rarely occurs in the perianal region. Recently, we experienced a case of perianal extramammary Paget's disease associated with an anal duct adenocarcinoma. The patient was a 60-year-old man. The perianal skin lesion was eczematous and encircled the anus. A wide-excision, split-thickness skin graft and temporal T-loop colostomy were performed. Histopathologically, the tumor was a well-differentiated anal duct adenocarcinoma. There was a prominent pagetoid spread of about 6x4 cm. The tumor cell was positive for carcinoembryonic antigen, but the paget cell was negative. The patient was treated with radiation therapy and with single 5-FU chemotherapy six times. Five months later, the perianal region was nearly normal.