Twenty-six consecutive admissions of 24 patients with severe ulcerative colitis (UC) hospitalized in our Department at some time between January 1983 and December 1988 were studied to identify factors useful in the prediction of response to medical treatment in the acute inflammatory phase of this disease. Results of laboratory tests (white blood cells, red blood cells, platelet count, hemoglobin, erythrocyte sedimentation rate, total protein, albumin, alpha 2-microglobulin, cholinesterase, total cholesterol, and triglycerides) and of endoscopic findings (extent of disease, progress of the lesions, sparing of the rectum, and presence of geographic ulcers, longitudinal ulcers, and polypoid mucosal tags) were analyzed for any relationship with the effect of medical treatment during the acute phase. The effect of treatment was evaluated in terms of days it took for a severe condition to improve to an intermediate one defined by Truelove and Witts' categories for UC severity. C-Reactive protein, nutritive condition (total protein, albumin, and cholinesterase), extent of the lesions, and existence of polypoid mucosal tags provide predictive factors useful in the management of UC during the acute phase.