Light micrograph of chronic inflammation of ulcerative colitis. Ulcerative colitis is inflammation of the bowel, typically arising in the rectum and often extending to involve part of, or most of, the large intestine. This form of colitis is characterized by deep ulcers, which are usually superficial but may perforate the bowel wall in some cases. The affected bowel lining becomes hemorrhagic, contributing to diarrhea and the passing of blood and mucus.