Mucinous adenocarcinoma developing in a chronic anal fistula is a rare tumor of the anus of which there are less than 150 reported cases. There has been some debate as to whether the fistula is the source of the tumor, or whether the fistula is the presenting feature of a slow-growing, indolent carcinoma. Two recent cases seen at our hospital are presented, along with a review of the literature and what we feel to be strong evidence that the fistula and associated anal glands are indeed the source of this unusual tumor.