Three cases of sarcomatoid carcinoma of the small intestine are presented. One of them was found accidentally in the duodenum of a patient with a well differentiated adenocarcinoma and a malignant lymphoma that were limited to the stomach. The other two cases arose from the ileum. All of the tumors were whitish, soft and ulcerated with focal hemorrhage and necrosis and showed expansive growth. Each tumor consisted of a mixture of polygonal and spindle shaped anaplastic neoplastic cells arranged in sheet, short fascicular or haphazard fashion, with no finding suggesting epithelial differentiation. Special stains demonstrated intracellular mucin in only a small number of tumor cells in two cases, but not in the other case. Immunohistochemically, the tumor cells of two cases at both primary and metastatic sites showed a positive immunoreaction for cytokeratin and epithelial membrane antigen. In the other case, only a few tumor cells at the metastatic site, but not at the primary site, showed cytokeratin positivity. Various numbers of tumor cells positive for vimentin, alpha-1-antitrypsin (AAT), alpha-1-antichymotrypsin (ACT) and KP-1 were detected in each case. Ultrastructurally, some populations of tumor cells possessed various amounts of tonofilaments with a few intercellular connections between adjacent tumor cells. These cases should be classified as sarcomatoid carcinoma of the small intestine, despite partial or complete loss of epithelial features, and distinguished from the various sarcomas.