The risk of colorectal cancer for any patient with ulcerative colitis is estimated to be 2% after 10 years, 8% after 20 years and 18% after 30 years of disease. The relative risk of colorectal cancer in Crohn's colitis is approximately 5.6 and should raise the same concerns as in ulcerative colitis. Risk factors for colorectal cancer include disease duration, early onset, extensive disease, primary sclerosing cholangitis and a family history of sporadic colorectal cancer. All patients should have a review colonoscopy 8-10 years after diagnosis to establish the extent of the disease. Surveillance should begin 8-10 years after disease onset for pancolitis and 15-20 years after disease onset for left-sided disease. Regular surveillance is recommended, with a screening interval every 3 years in the second decade of disease and annually by the fourth decade. Random biopsies should be taken at regular intervals with attention paid to dysplasia-associated lesions or masses, irregular plaques, villiform elevations, ulcers and strictures. Dysplasia is recognized as a premalignant condition, but the likelihood of progression to cancer is difficult to predict. High-grade dysplasia, confirmed by two expert gastrointestinal pathologists, is a strong indication for colectomy, as is low-grade dysplasia, although the diagnosis of low-grade dysplasia is unreliable. Surveillance programmes indicate that the overall 5-year survival rate is higher in surveyed patients, although patients still present with Dukes C cancers or disseminated malignancy. Surveillance has huge socioeconomic implications. As surveillance is not 100% effective, alternative ways of reducing the cancer risk with chemopreventive agents, such as aminosalicylates, are being considered.