Individuals with chronic ulcerative colitis are at increased risk of developing colorectal carcinoma, particularly if there is long-standing disease or extensive colitis. It is generally accepted that the risk of colorectal cancer does not begin until 8 to 10 years after the time of diagnosis of ulcerative colitis. Thereafter it increases by approximately 0.5% to 1.0% per year. In patients with Crohn's disease, the risk of malignancy is smaller and less well defined. The most significant predictor of the risk of malignancy in patients with inflammatory bowel disease is the presence of dysplasia in colonic biopsies. There is considerable controversy in the literature regarding the efficacy of colonoscopic surveillance programs and the role of prophylactic surgery to prevent colorectal cancer. Surveillance certainly fails to detect carcinoma in some patients who are having regular colonoscopy. Concerns have also been raised as to the cost-benefit of colonoscopic surveillance in patients with colitis. Randomized controlled trials of surveillance programs are highly unlikely in view of the low prevalence of IBD in the population, the long period of observation required, and the probability of contamination of surveillance programs by colonoscopy for assessment of disease activity. Despite the lack of clear guidelines, surveillance colonoscopy and biopsy continues to be widely practiced. Research is proceeding to identify genetic and biochemical markers that may prove clinically useful for predicting cancer risk. At present, however, surveillance programs are likely to continue according to institutional practice. It is important for those participating in such programs to be aware of the limitations of colonoscopy and biopsy as a means of reducing the risk of cancer in inflammatory bowel disease.