A 53-year-old man presented with cerebral infarction associated with a dissecting aneurysm of the left middle cerebral artery (MCA), with enlargement and fluid collection. Anticoagulant therapy was performed as the first stage treatment for cerebral infarction. Serial magnetic resonance imaging showed that the dissecting aneurysm had enlarged and fluid collection adjacent to the aneurysm had developed since the first admission. Surgery was performed to ligate the MCA proximal to the aneurysm. Intraoperative findings showed the branch of the MCA was obstructed by intramural hemorrhage of the aneurysm dome. Histological examination showed direct obstruction of the MCA branch artery by intramural hemorrhage of aneurysm dome had caused the cerebral infarction and the fluid collection surrounding the aneurysm resulted from minor leakage or exudation of intramural hemorrhage to the outer surface of the dissecting aneurysm.