Ulcerative colitis is an inflammatory disease of the colon or large bowel.
The inflammation usually only involves the inner lining of mucosa of the colon and does not extend into the muscle coat. The inflammation consists of fine ulceration of the surface with infiltration of the involved area by special types of inflammatory white blood cells from the blood. Characteristically, the inflammation starts at the lower end of the colon, just above the anus and may extend upwards a variable distance. In some patients only a short distance is affected and in others it may involve the whole colon. Ulcerative colitis is not related to ulcers found elsewhere in the gastro-intestinal tract such as stomach ulcers or duodenal ulcers, but it has many similarities to another chronic inflammatory disease of the bowel, Crohn's disease.