A 33-year old female with a 10-year history of ulcerative colitis developed a large polypoid filling defect in the distal transverse colon, suggestive of carcinoma of the colon. Biopsy indicated an inflammatory polyp but subtotal colectomy was performed because of the long history and the radiographic appearance. Pathologic examination revealed an unusual aggregation of filiform polyps entrapping a semisolid fecal mass. The development of this unique lesion is elaborated.