According to Truelove and Witts, ulcerative colitis has been rated only by clinical classification without taking into account morphological alterations, and so far (in contrast to Crohn's disease) no activity index has been available for clinical studies. Therefore, we have developed an index during a therapeutic trial with mesalazine (5-aminosalicylic acid) to evaluate the initial state of inflammation and assess therapeutic efficacy. The activity index includes 5 items: stool frequency (score 0 to 3), rectal bleeding (score 0 to 3), endoscopy (score 0 to 3), histology (score 0 to 4), and extension of inflammation (score 0 to 4). In 42 patients with ulcerative colitis treatment with mesalazine (2 suppositories of 250 mg 3 times daily) was monitored for 12 weeks. In 37 patients a clinical improvement was observed, as indicated by a significance decrease in the mean activity index from initially 10.2 to 6.1 (after 6 weeks) and 3.4 (after 12 weeks). The proposed index could be modified by additional parameters.