Ulcerative colitis is an inflammatory bowel disease (IBD) in which the lining of the large intestine (colon or bowel) and the rectum become inflamed. This inflammation produces tiny sores or ulcers on the lining of the colon. It usually begins in the rectum and spreads upward. It rarely affects the small intestine beyond the lower portion.

The inflammation causes the bowel to move bowel contents rapidly and empty frequently. As cells on the surface of the lining of the bowel die, ulcers (open sores) form. The ulcers may cause bleeding and discharge of mucus and pus.