A comparison between histologic and colonoscopic extension of ulcerative colitis was made in 107 examinations (83 patients). During colonoscopy signs of inflammation were registered, and biopsy specimens were taken from the following gut segments: rectum, left colon, transverse colon, and right colon. Inflammatory activity in the specimens was graded in accordance with severity in a scale from one to five. Endoscopically, total colitis was seen in 40 examinations but was present in 70 examinations histologically. In 34 of 107 examinations the extension of disease was underestimated at colonoscopy. A slight inflammation existed in those segments that appeared normal colonoscopically. Our conclusion is that the extension of ulcerative colitis often is underestimated endoscopically and that inflammatory activity can be present in mucosa assessed as normal on colonoscopy.