Quantification of disease severity was studied in 72 patients with ulcerative colitis, who had undergone total 85 clinical courses. We performed a multiple stepwise regression analysis, according to Truelove and Witts' classification, with disease severity as a dependent variable, and with 18 clinical, laboratory, and endoscopic parameters as independent variables. Results showed that disease severity in patients with ulcerative colitis is significantly influenced by five factors, namely, bloody stool, bowel movements, erythrocyte sedimentation rate (ESR), hemoglobin (Hb), and serum albumin. The activity index (AI) developed for ulcerative colitis is expressed as follows: AI = 60 x blood stool + 13 x bowel movements + 0.5 x ESR - 4 x HB - 15 x albumin + 200. Index values below 150, values between 150 and 220, and values above 220 nearly corresponded to mild, moderate, and severe disease, respectively, in Truelove and Witts' classification. We believe that the activity index is useful in evaluation of the effect of medical treatment in patients with ulcerative colitis. Its most important value will be in therapeutic trials.