BACKGROUND Colorectal cancer that develops as a complication of ulcerative colitis (UC) is a serious problem that affects the patient's prognosis. Such cancer is characterized by development at an early age, a high incidence of multiple tumors, poorly differentiated carcinoma and mucinous carcinoma. Special attention should therefore be paid to the diagnosis and treatment of such cancer. METHODS One hundred and seventy-four patients with UC underwent surgery in our Department between July of 1985 and December of 2009. Of these, 22 had concomitant colorectal cancer. We performed a retrospective study to investigate these patients. RESULTS The incidence of colorectal cancer as a complication of UC was 12.6%. The male:female ratio was 14:8, and the average age at surgery was 54.6 (32-79) years. In addition, when examining the lesion type of UC, it was revealed that the total colitis type accounted for 77.3% of colorectal cancer cases in UC patients. Regarding the site of development of colorectal cancer, 14 out of the 22 patients had cancer in the distal end. The average period from the development of UC to the diagnosis of colorectal cancer was 14.7 (0.6-40.5) years. The cumulative incidence rates over 10 and 20 years were 5.1% and 17.5%, respectively. Histologically, poorly differentiated adenocarcinoma and mucinous carcinoma were confirmed in 38.1% of the patients, and dysplasia was also confirmed in 53.8%. In addition, multiple tumors were confirmed at a rate as high as 27.3%. Cancer detection through surveillance has increased, and colorectal cancer was detected in 13 out of the 22 patients by routine surveillance. In cases where cancer was detected by surveillance colonoscopy, 46.2% of lesions were early cancer. We therefore consider that surveillance is useful. However, we experienced a case that could not be diagnosed by endoscopy that was successfully diagnosed by fluoroscopy. The case was noted to have stricture. CONCLUSIONS The cumulative incidence rates over 10 and 20 years were 5.1% and 17.5%, respectively. Since the average period from the onset of UC to the diagnosis of colorectal cancer was 14.7 years, routine surveillance examinations are necessary for patients with a history of UC of at least 10 years. In addition, patients with strictures must be examined using both colonoscopy and fluoroscopy because diagnosis with colonoscopy alone may be inadequate.