Perianal mucinous adenocarcinoma, also known as anal duct or anal gland carcinoma, is a rare tumor usually associated with chronic fistulae-in-ano. Whether the tumor occurs primarily and the fistula is a secondary manifestation, or whether this tumor arises in a chronic fistulous tract is debatable. Diagnosis can be made only by open deep biopsy of the buttock mass, and, due to the low grade histologic appearance of the malignancy, an incorrect diagnosis is frequently made. Treatment is abdominoperineal resection with wide removal of the buttock mass.