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Patients at highest risk for developing cancer in ulcerative colitis are those with ‘extensive’ or total involvement of the large bowel who have had the disease for at least seven years. Dysplasia is used as a marker bur has many problems including those of sampling, reproducibility and management. The risk in patients with colitis is unclear particularly in those with left-sided or distal ulcerative colitis. In countries at high risk from colorectal cancer about 4 to 6% of the population can be expected to develop this disease. It is assumed that surveillance will reduce the mortality from colorectal cancer, although the evidence that this is happening is very limited. Cancers which are resected but from which the patient survives are an acceptable outcome, although less so in theory, as survival is to a certain extent fortuitous. Many surveillance studies include patients who have both developed and died from carcinoma. Surveillance also assumes that cancers can be detected before they have become lethal, or that a marker such as the presence of dysplasia precedes all carcinomas for a long enough period of time to be detectable. Considerable question has been raised as to whether dysplasia is both endoscopically detectable and morphologically identifiable. Surveillance is based on the principle that carcinoma arises from a cancerous lesion, and that the identification of dysplasia and excision of the large bowel in these patients prevents subsequent death from disseminated carcinoma. Conversely, patients with quiescent disease and no dysplasia could be followed and not subjected to unnecessary colectomy. There is currently no ‘best’ way of managing patients with colitis who are at risk for developing carcinoma. Routine follow-up of patients relies heavily on colonoscopy with multiple biopsies. Controversy continues regarding the management of dysplastic biopsies because there are relatively few data regarding the likelihood of an underlying invasive carcinoma on which to base a rational decision. The notion that all patients must be managed on an individual basis, guarantees that data remain difficult to obtain. The presence of a dysplasia-associated lesion or mass are high risk factors for carcinoma. Dysplasia is frequently confined to small areas of the mucosa causing major sampling problems for the endoscopist both in detection and if confirmation by re-endoscopy is proposed. The finding of aneuploidy as a marker for both dysplasia and carcinoma may prove useful in the detection of patients at greatest risk.