Blood may appear in the stool because of one or more of the following conditions: Benign (non-cancerous) or malignant (cancerous) growths or polyps of the colon Hemorrhoids (swollen blood vessels near the anus and lower rectum that can rupture, causing bleeding) Anal fissures (splits or cracks in the lining of the anal opening) Intestinal infections that cause inflammation Ulcers Ulcerative colitis Crohn s disease Diverticular disease, caused by outpouchings of the colon wall Abnormalities of the blood vessels in the large intestine Gastrointestinal bleeding may be microscopic (invisible to the eye) or may be easily seen as red blood or black tar-like bowel movements, called melena.