The macroscopic and histological appearance, and the local immune response in ulcerative colitis are discussed. The main criteria for the differentiation between ulcerative colitis and Crohn's disease of the large bowel are reviewed. The risk to develope carcinoma in the large bowel is greater in patients with total ulcerative colitis than in the general population. Precancerous changes in rectal and colonoscopical biopsies are a useful parameter in detecting early cancer in colitis. A description of the morphology of precancerous changes in ulcerative colitis is given.