Endometriosis is the abnormal growth of endometrial tissue outside of uterine cavity. The incidence of endometriosis is estimated about 8∼15% of the reproductive women. The intestinal invasion is showed at about 3∼34%. The intestinal invasion may be involved in rectosigmoid colon, appedix, ileum, and cecum in order of decreasing frequency. Until now, only 7 cases have been reported in Korea. Most cases are diagnosed after surgery. And then the diagnosis need to be confirmed by surgical pathology. Recently, a 42 year-old woman visited due to severe constipation. A flexible sigmoidoscopy revealed submucosal tumor on rectosigmoidal juction 20 cm above the anal verge. The surgical resection was performed and the biopsy showed the endomtriosis involving mucosa, submucosa, muscularis, serosa, subserosa and lymph node. (Korean J Gastrointest Endosc 2004;29:534538)