It has been widely assumed that the activity of ulcerative colitis is related to differences in mucosal appearances seen at sigmoidoscopic examination. We have tested this assumption by making comparisons of the symptoms and clinical signs associated with three reproducible appearances of the rectal mucosa. By cross-tabulating 222 observations of each of 10 symptoms and signs with these sigmoidoscopic appearances it has been shown that the subdivision of hemorrhagic mucosae into those which bleed spontaneously and those which bleed only on light touching or scraping is meaningful clinically. Sigmoidoscopic appearances seemed to correlate better with clinical disease activity than histological assessment, even when quantitative, of mucosal biopsies. On the basis of this study, four variables have been suggested which, in addition to the sigmoidoscopic appearances, could form the basis of regular clinical assessment or scoring.