A 32-year-old Japanese woman with a 14-month history of ulcerative colitis (UC), pancolitis type, was referred to our institution, because of abdominal distention. Plain abdominal X-ray and computed tomography (CT) showed marked dilatation of the right side of the colon. The patient was treated by immediate total colectomy, with the diagnosis of toxic megacolon. Macroscopically, there was a constricting lesion with giant polyposis in the middle part of the transverse colon. Histologically, there was transmural inflammation with a thickened proper muscular layer overlaid by inflammatory polyposis. This case suggests that giant polyposis in UC patients may result in severe stenosis and that such a condition should not be misinterpreted as toxic megacolon or as colitic cancer.