Of approximately 1,500 cases of primary ulcerative disease of the colon seen at the Cleveland Clinic since 1950, about half may be classified as regional enteritis of the colon. Grossly, the disorder is characterized by serpiginous or longitudinal furrow ulcers or both, segmentally or diffusely arranged with areas of normal-appearing mucosa separating the ulcers. Microscopically, the inflammation is transmural. Noncaseating, sarcoid granulomas may be found but are absent in one third of the cases. Deep fissures, which may extend through the bowel wall, may be observed in 25% to 30% of cases and appear to be the basis for fistula formation. Post-colectomy ileitis occurs far more commonly in patients with regional enteritis of the colon than in patients with chronic ulcerative colitis; the incidence is about equal in patients with or without ileitis at the time of first colectomy. No increased risk of the development of carcinoma was noted.