Neurosyphilis often develops brain infarction. Recently, the authors experienced a case of brain infarction caused by syphilitic aneurysm on the thoracic aorta, and its clinical significance is reported. A seventy-eight-year-old woman with strongly positive serologic tests for syphilis suddenly developed global aphasia and right hemiplegia. Brain computed tomography (CT) showed a large brain infarction in the left middle cerebral artery territory. A huge aneurysm of the ascending aorta eroded the sternum and the second and third ribs. The left common carotid artery was completely compressed by the aneurysm, and blood flow was not demonstrated on enhanced CT. A cerebrospinal fluid examination revealed normal cell counts, and the titer of a syphilis test was very low. She did not have any arrhythmia, ischemic heart disease, or valvular heart disease as an embolic source. Brain infarction may be developed by pressure of an aortic aneurysm on the left common carotid artery. They recommend consideration of syphilis as an etiology of brain infarction when luetic patients are seen in old age.