Blood in the stool can be caused by bright red bleeding coming from hemorrhoids, anal fissures, fistulae or cancer of the rectum and anus. Bleeding can also come from diverticular problems in the colon, colon cancer and colonic polyps etc. Colitis or gastroenteritis can also cause bleeding that needs to be checked. Upper gastrointestinal bleeding can come from the stomach or small bowel also. Mostly bleeding from these sources is dark brown or black. These symptoms should not be ignored and patients should seek medical help soon.