The first case of an adenocarcinoma developing in the retained anal canal mucosa (transitional zone) after restorative proctocolectomy with a stapled ileal pouch anal anastomosis is presented. The cancer was detected during routine follow-up 16 months after pouch formation for long standing ulcerative colitis, complicated by a cancer in the upper rectum. The patient was treated with an abdominoperineal excision of the ileal pouch and anus.