Twenty-six radiological, endoscopic and histological examinations of the large bowel were performed in 25 patients with ulcerative colitis. Extensive colitis was observed in 42% of the radiological, in 38% of the endoscopic, and in 27% of the histological examinations. Agreement as to the extent of the colitis between all three examination methods was reached in 53% of the cases. The endoscopic extent of colitis varied between left-sided and extensive in 41% of the patients during a 7-year median follow-up. The frequent change in the extent of ulcerative colitis and the considerable disagreement between radiography, colonoscopy and histology in evaluating the extent of colitis justify the question, "Does a true, unambiguous extent of colitis really exist?" It is more likely that the extent of colitis is just a function of time and the examination method used.