In the years 1966-80, 303 patients with radiologically extensive ulcerative colitis were followed up regularly, both by clinical assessment and by mucosal biopsies, to detect dysplastic (precancerous) change. Moderate or severe dysplasia (but no carcinoma) was found in the specimen in 8 patients treated surgically. Despite regular surveillance, carcinoma developed in 13 of the 186 patients with a history of disease for ten years or more. Of the total sixteen carcinomas in these 13 patients, eleven were Dukes stage A, three stage B, one stage C, and one was inoperable. There has been 1 death from carcinoma in this series over the fifteen year observation period. The method of follow-up has probably prevented the development of carcinoma in 8 patients and permitted diagnosis of carcinoma at a stage when cure is likely in 11. The incidence of moderate or severe dysplasia and carcinoma in these patients with mild or symptomless extensive colitis shows that either proctocolectomy or careful follow-up should be advised once the duration of symptoms reaches ten years.