Ulcerative colitis is a chronic inflammation of the large intestine (colon). The colon is the part of the digestive system where water is removed from undigested material, and the remaining waste material is stored. The rectum is the end of the colon adjacent to the anus. In patients with ulcerative colitis, ulcers and inflammation of the inner lining of the colon lead to symptoms of abdominal pain, diarrhea, and rectal bleeding.
Ulcerative colitis is closely related to another condition of inflammation of the intestines called Crohn's disease. Together, they are frequently referred to as inflammatory bowel disease (IBD). Ulcerative colitis and Crohn's diseases are chronic conditions that can last years to decades. Men and women are affected equally. They most commonly begin during adolescence and early adulthood, but they also can begin during childhood and later in life.

It is found worldwide, but is most common in the United States, England, and northern Europe. It is especially common in people of Jewish descent. Ulcerative colitis is rarely seen in Eastern Europe, Asia, and South America, and is rare in the black population. For unknown reasons, an increased frequency of this condition has been observed recently in developing nations.