Familial adenomatous polyposis(FAP) is a rare hereditary disorder characterized by the development of hundreds to thounds polyps throughout the colon and rectum. Moreover, several extracolonic manifestations are seen. Recently, this disease is recognized as a adenomatous polyposis syndrome which can involve the entire astrointestinal tract. Several reports have demonstrated a high incidence of gastroduodenal polyps in patients with familial adenomatous polyposis. These colon polyps can be eventually developed as colon cancer, if not be treated. So early diagnosis is needed and prophylactic surgery should be erformed. We experienced a case of familial adenomatous polyposis with a large sentinel polyp on the sigmoid colon, presenting hematochezia and mucoid diarrhea. He was early treated before progression to carcinoma by total colectomy, rectal mucosectomy and J pouch ileoanal anastomosis. (Kor J Gastrointest Endosc 15: 775-782, 1995)