The occurrence of pseudomalignant ulcerative change in seven specimens from the colon and rectum of six patients is described. In all cases, there was surface ulceration of a polypoid lesion which contained granulation tissue and acute and chronic inflammation. There was an underlying inflammatory pseudopolyp in four lesions, a juvenile polyp in one lesion, an adenomatous polyp in one lesion, and a benign retention polyp in one lesion. Within the stroma of all cases were numerous atypical cells that mimicked a malignant neoplasm. The atypical cells expressed vimentin in immunohistochemical studies; no expression of keratins, leukocyte common antigen, factor VIII, Ulex europaeus, carcinoembryonic antigen, actin, or desmin was found. Recognition of this lesion is important, as confusion with carcinoma, lymphoma, sarcoma, or a viral infection may easily occur. © 1991.
