Mucinous adenocarcinoma arising in an anorectal fistula is an uncommon condition which gives rise to difficult problems of diagnosis and pathogenesis. The clinical history and pathology of seven patients are described and compared with six patients in whom anal fistulae were lined by normal rectal mucosa or ‘misplaced glands’. In five of the cases granulomas were present which were a further cause of diagnostic difficulty. The evidence from this study suggests that the fistulous tracks are congenital duplications of the lower end of the hind gut lined by rectal mucosa which is prone to malignant change to mucinous adenocarcinoma. The prognosis after excision of the rectum is good.