Ulcerative colitis is one of the least known yet most challenging problems of modern medicine. Though much less common than cancer, heart disease, or peptic ulcer, ulcerative colitis, nevertheless, is more frequent than is generally appreciated. It has been suggested also that the disease is increasing in frequency. Though differences in incidence are thought to exist between countries and continents, the geographic distribution of ulcerative colitis requires further study. A curious and unexplained feature is the apparent infrequency of ulcerative colitis in city or country hospitals contrasted with private institutions. Ulcerative colitis is not epidemic, infective, or transmissible. There is no satisfactory evidence of inherited susceptibility to the disease. Its presence in more than one member of a family is uncommon, though it occurs. Young people between the ages of twenty and forty and children are affected most often, but the disease may develop at all ages. It is commoner