Serial measurements of 11 serum proteins have been made throughout 39 admissions of 36 patients to hospital for the treatment of acute attacks of ulcerative colitis. There was a striking correlation between rapid changes in C-reactive protein and pre-albumin concentrations and the clinical response to medical treatment. Measurements of the alpha1-acid glycoprotein, albumin, and total serum protein concentrations at the time of admission were found to correlate with the outcome of the attack. Measurement of these proteins provides a useful guide to the management of patients with attacks of ulcerative colitis.