Endometriosis of the rectum is unusual condition, since it represents an invasion of previously normal bowel by hormone-dependent nonmalignant cell from uterus of the same patient. It is estimated that the incidence of endometriosis is about 8-15% of reproductive women, of whom 3-34% show intestinal invasion of rectosigmoid colon, appendix, ileum, and cecum in order of decreasing frequency. Symptoms related bowel involvement may vary from none to complete intestinal obstruction. Because the mucosa is involved infrequently there is rarely rectal bleeding. X-ray and sigmoidoscopic studies are usually negative. It was not reported that colonic endometriosis was confirmed by endoscopic biopsy. Recently we experienced a case of rectal endometriosis which presented itself as a cyclic rectal bleeding with abdominal pain and diagnosed by colonoscopic biopsy.