Patients with ulcerative colitis (UC) have the increased risk of colorectal cancer. The risk of colorectal cancer development in UC is associated with the extent of colitis and duration of disease. It is generally agreed that there is little or no increased risk associated with proctitis or proctosigmoiditis. A dysplasia-associated lesion or mass (DALM) is difficult to distinguish from sporadic adenoma. The DALM indicate a high association with colon cancer. The presence of DALM as a premalignant lesion has been considered as an indication for colectomy because of the high risk of progression to malignancy within a short period. We report a case of polyp with high grade dysplasia in a 27-year-old man of ulcerative colitis confined to rectum and sigmoid colon. (Korean J Gastrointest Endosc 2004;29: 4649)