A 56-year-old man was hospitalized because of swelling of the right upper extremity and anemia. A diagnosis of superior vena cava (SVC) syndrome caused by lymphogenous metastasis was made after chest computed tomography (CT) scan and biopsy of cervical lymph nodes were carried out. Standard examinations, such as abdominal CT scan and endoscopies of the upper and lower gastrointestinal tract, failed to find the primary lesion. However, selective angiography of the superior mesenteric artery (SMA) showed a clear stain of bleeding vessels in the small intestine. Laparotomy was performed, and immunohistochemical findings revealed sarcomatoid carcinoma in the small intestine (a rarely seen neoplasm). This aggressive carcinoma, which showed negative reactivity with CD34, CD117 (c-kit), and S-100 was clearly distinguished from other mesenchymal tumors, such as malignant gastrointestinal stromal tumor (GIST) and malignant fibrous histiocytoma (MFH).