A 79-year-old man with a 2-month history of fever and weight loss was admitted to our hospital because of an acute abdomen. Abdominal CT scans showed marked sectional thickening and edema of the small intestine. On laparotomy, a 16-cm section of the small intestine was ischemic and necrotic; therefore, segmentectomy of the intestine was performed. A thrombus was noted at the stump of the mesenteric artery branch. Histopathological analysis of the resected intestine revealed fibrin thrombi in both mesenteric arteries and veins. Furthermore, a cluster of large, abnormal lymphoid cells bordering the intima of most branches of the mesenteric veins and small vessels was observed. Immunohistochemical analysis revealed that these abnormal cells were positive for CD20, leading to a diagnosis of intravascular large B-cell lymphoma (IVLBCL). The patient was successfully treated with standard R-CHOP chemotherapy; however, the lymphoma recurred in the central nervous system 18 months after the initial diagnosis, and the patient died. Simultaneous thrombosis of the mesenteric artery and vein is unusual as a clinical manifestation of IVLBCL. However, IVLBCL should be taken into consideration when ischemic disorders of unknown cause, accompanied by fever of unknown origin, are encountered.