Since the original report of Crohn and Rosenberg(1928) the association between long-standing ulcerative colitis and later development of colonic malignancy has been well known. There are many risk factors for the development of malignacy such as duration extent severity and age of the onset of ulcerative colitis drugs and diagnostic radiation. the dysplasia of the colonic mucosa as a precancerous change are seen not only in the area of malignant focus but also in distant locations in long-standing ulcerative colitis in the rectosigmoid junction. This is probably the first documentation of malignant transformation of ulcerative colitis in the rectosigmoid junction. This is probably the first documentation of malignat transformation of ulcerative colitis in the Korean literature.