The authors describe a very rare case of intravascular large B-cell lymphoma in a woman whose ruptured distal middle cerebral artery (MCA) aneurysms were filled with lymphoma cells. A 69-year-old woman who had undergone artificial graft replacement for an aortic aneurysm presented with transient left hemiparesis. Magnetic resonance imaging demonstrated a small fresh cerebral infarction in the right frontal lobe, although major cervical and cerebral arteries were shown to be intact on MR angiography. Antiplatelet and anticoagulation treatments commenced. On the 21st day after onset, the patient suffered a subarachnoid hemorrhage, and a digital subtraction angiogram revealed aneurysmal lesions in the distal MCA. Based on the histological examination of the resected aneurysms, proliferation of large B-cell lymphoma was identified in the dilated arterial lumen. On the 71st day after ischemic onset, intracranial hemorrhage recurred, and she died. Postmortem examination revealed similar lymphoma cells only in the intimal layer that had grown on the artificial graft, and it was decided that the patient had had intravascular large B-cell lymphoma. The preceding cerebral infarction was thought to be due to occlusion of the distal MCA by tumor embolus, which may be the initial pathological stage in aneurysm formation. For patients with incomprehensible ischemic cerebral stroke, neoplasm must be taken in consideration.