True annular malignancies of the small bowel with mucosal destruction and shelflike margins are generally thought to be caused by primary adenocarcinoma. At our institution, 18 annular malignancies were diagnosed radiographically in the small bowel by enteroclysis (16 cases) and conventional small bowel follow-through studies (2 cases) between 1977 and 1984. However, pathologic data revealed only 4 primary adenocarcinomas with 10 metastatic lesions (6 colon cancers, 2 malignant melanomas, 1 lung cancer, and 1 cervical cancer), 2 leiomyosarcomas, 1 non-Hodgkin's lymphoma, and 1 malignant carcinoid tumor. While these lesions may be indistinguishable radiographically, annular carcinomas tended to be short, relatively nonobstructing lesions; annular metastases (except those from malignant melanoma) tended to be highly obstructing lesions with significant narrowing and/or angulation of the bowel. Leiomyosarcomas, lymphoma, and metastases from malignant melanoma tended to be longer lesions with extensive ulceration, wider channels, and little or no evidence of obstruction. Nevertheless, surgical resection or biopsy of the lesion is ultimately required for a definitive diagnosis.