A series of 150 patients with ulcerative colitis, 100 of which had had the disease more than 10 years, underwent colonoscopy on at least one occasion. 4700 biopsies of the mucosa of the gut were taken and examined histopathologically. 148 of the colonoscopies were performed within 12 months of the radiographic examination of the colon. Precancerous lesions were detected in 12 patients and moderate to severe reactive inflammatory changes in 39. One of the 12 patients with precancerous lesions was found to have a carcinoma in the same region at surgery 18 months later. No further cases of carcinoma in the series have been observed at follow-up surgery. Radiography and endoscopy demonstrated a higher rate of certain features of inflammation in the parts of colon with precancerous lesions, but no particular feature could be taken to indicate the presence of such lesions. All patients with precancerous lesions had on some occasion been radiologically and/or endoscopically judged to have total colitis, compared with 75% in the whole series. Analysis of clinical data in patients with and without precancerous lesions revealed no significant difference between the groups.