Proctitis cystica profunda is an uncommon benign rectal condition characterized by bloody stool and/or mucus, diarrhea, tenesmus, abdominal pain, or rectal pain. Since thickened muscularis mucosa and misplaced hyperplastic glands containing pools of mucin into the submucosa are pathologically typical features, it is easily diagnosed. Physical findings are varied, and include palpable mass, polyp, or irregular mucosal surface on digital rectal examination. On endoscopic examination, polyps, sub- mucosal cysts, ulcerations, or irregular villous masses may be observed. Since it could be mistaken for malignant tumor both grossly and microscopically, it is important to differenciate proctitis cystica profunda from malignant tumor. We present an unusual case of proctitis cystica profunda diagnosed by endoscopic mucosal resection. It has been previously interpreted as adenomatous polyp by simple biopsy. (Kor J Gastroenterol 1998;32:536 - 540)