 In contrast to Crohn's disease, which can affect areas of the gastrointestinal tract outside of the colon, ulcerative colitis is usually confined to the colon.Inflammation in ulcerative colitis is usually continuous, typically involving the rectum, with involvement extending proximally (to sigmoid colon, ascending colon, etc).In contrast, inflammation with Crohn's disease is often patchy, with so-called "skip lesions."