In a 16-year series of 235 patients operated on for ulcerative colitis (UC), 16 patients had altogether 21 colorectal malignancies: 15 patients adenocarcinomas and 1 a lymphoma. These patients were analyzed to find the typical features of colitic cancer and to evaluate the chance for early detection of cancer. The mean age of the patients was 35 years (19-86), and the mean duration of UC 14 years (5-26). All patients had total colitis, and 13 of 21 tumours (62%) were located proximal to the splenic flexure. Three patients under regular surveillance had Dukes' stage A cancers, while those with only initial or no follow-up had more advanced tumours (1 Dukes' A, B, 1C, 10D). Early cancer was symptomless or indistinguishable from colitic symptoms. The reasons for inadequate surveillance were few symptoms of UC (n = 9), an unsatisfactory follow-up system (n = 3) and patient refusal (n = 1). Our survey indicates that only regular follow-up can disclose early tumours and, thus, improve the prognosis of colitic carcinoma. The necessity of establishing colonoscopic surveillance of UC patients soon after the diagnosis, and regardless of the degree of symptoms is emphasized, because 7 (44%) of our patients developed cancer within 10 years of the onset of UC.