Eight patients with ulcerative colitis, three with colitis indeterminate, and 15 patients with Crohn disease were studied by computed tomography (CT) to establish CT criteria for each disorder in hopes of providing a new diagnostic perspective useful in the radiographic evaluation of inflammatory colitis. The CT findings in ulcerative colitis included thickening of the colon wall (mean, 7.8 mm), which was characterized by inhomogeneous attenuation and a "target" appearance of the rectum, and proliferation of perirectal fat. Bowel wall thickening (mean, 13 mm) with homogeneous attenuation, fistula and abscess formation, and mesenteric abnormalities were observed in patients with Crohn colitis. Patients with colitis indeterminate showed colonic changes on CT observed in both disorders. Initial experience suggests that CT can differentiate patients with well established ulcerative and Crohn colitis.