Colon cancer is a well known complication of ulcerative colitis. Various methods have been discussed to reduce its risk. Since the introduction of the concept epithelial dysplasia and fiberoptic colonoscopy surveillance programmes have been launched based on these methods. The incidence of colon cancer in ulcerative colitis is moderately increased but constitutes only a fraction of all colon cancers diagnosed. Among these patients colon cancer almost exclusively affects those with extensive colitis. Duration of disease in these cases usually exceeds eight to ten years. The true benefit of a cancer surveillance programme will probably never be the subject of controlled prospective studies and is therefore difficult to evaluate. The concept of dysplasia is the major instrument for selecting patients for prophylactic colectomy. Development from normal mucosa to high grades of dysplasia shows individual variations from one to several years justifying annual colonoscopies in high risk patients. Strict criteria for dysplasia and independent evaluations from several pathologists should compensate for the subjective nature of dysplasia interpretation. The need for more objective markers such as DNA-flow cytometry is clearly needed. The role of DNA flow cytometry in surveillance, however, needs to be determined. The cost benefit aspects of such surveillance programmes have to be balanced against the total resources of a gastroenterology unit.