Objective. Patients with ulcerative colitis (UC) have an increased risk of developing colorectal cancer (CRC). The current procedure to diminish this risk is colonoscopic surveillance and histopathological evaluation of biopsy specimens. This method is not unquestioned and is undergoing continuous evaluation. The present study is a major update of an earlier reported investigation from an ongoing surveillance programme. Material and methods. In 1977 a colonoscopic surveillance programme comprising all patients with UC from a defined area was started in ? rnsk ldsvik. Three principal investigators performed regular colonoscopy with mucosal sampling for histopathological evaluation. Some 211 patients were studied from 1977 to 2002. At the end of the study period, 90 patients, including those operated on, had total colitis (TC) for more than 10 years, 74 patients had left the study, 31 after panproctocolectomy (PPC), 6 owing to advanced biological age, 1 because of intercurrent disease, 23 patients had moved out of the area and 13 patients were excluded because of poor compliance. In all, 928 colonoscopies were performed. Results. It was found that 135 patients had radiologically or morphologically defined TC and 69 patients had left-sided colitis (LC). Nine CRCs were diagnosed in 8 patients, one of whom died of CRC, while another two were included in the programme with a diagnosis of CRC. Morphological alterations classified as dysplasia or indefinite for dysplasia (ID) were found in 52 patients, 5 of whom were later found to have CRC. In total, 49 patients were operated on; in 15 of these patients the indication for surgery was dysplasia or CRC. Eighteen of the patients were operated on for different kinds of colonic resections and in 31 patients a PPC was performed. Conclusions. Colonoscopic surveillance is an effective method in preventing death from CRC among patients with UC. A conservative approach to surgery seems to be justified. The burden of the surveillance programme has been acceptable. The outcome depends on good patient compliance as well as the involvement of as few investigators as possible. Objective. Patients with ulcerative colitis (UC) have an increased risk of developing colorectal cancer (CRC). The current procedure to diminish this risk is colonoscopic surveillance and histopathological evaluation of biopsy specimens. This method is not unquestioned and is undergoing continuous evaluation. present study is a major update of an earlier reported investigation from an ongoing surveillance program. Some 211 patients were studied from 1977 to 2002. At the end of the study period, 90 patients, including those operated on, had total colitis (TC) for more than 10 years, 74 patients had left the study, 31 after panproctocolectomy (PPC), 6 owing to advanced biological age, 1 because of intercurrent disease , 23 patients had moved out of the area and 13 patients were excluded because of poor compliance. In all, 928 colonoscopies were performed. Results. It was found that 135 patients had radiologically or morphologically defined TC and 69 patients had left-sided colitis ( LC). Nine CRCs were diagnosed in 8 patients, one of whom died of CRC, while another two were included in the program with a diagnosis of CRC. Morphological alterations classified as dysplasia or indefinite for dysplasia (ID) were found in 52 patients, 5 of whom were later found to have CRC. In total, 49 patients were operated on; in 15 of these patients the indication for surgery was dysplasia or CRC. Eighteen of the patients were operated on for different kinds of colonic resections and in 31 patients a PPC was performed. Conclusions. Colonoscopy surveillance is an effective method in preventing death from CRC among patients with UC. A conservative approach to surgery seems to be justified. The burden of the surveilla nce program has been acceptable. The outcome depends on good patient compliance as well as the involvement of as few investigators as possible.