The case of a 59 year-old-man, with a history of relapsing anal suppurations lasting 27 years, affected by mucinous adenocarcinoma arising in perianal fistula is reported. This patient has no signs of relapsing cancer 28 months after preoperative radiotherapy, Miles operation and postoperative radiotherapy. Mucinous adenocarcinoma arising in anal fistula is rare; a delay in its diagnosis is almost constant and due to extramucosal growth: its prognosis can be ameliorating with an earlier diagnosis by performing systematic histological examination of all tissues withdrawn during proctologic surgery.