A persistent trigeminal artery (PTA) represents an embryonic vascular anastomosis connecting the carotid and basilar arterial systems. Little is known about its protective role in cases of basilar artery occlusion. We followed up a 63-year-old man who had suffered a brain stem infarction due to basilar artery stenosis and was found to have a PTA. Although a second brain stem infarction due to basilar artery occlusion developed, the circulation to the brain stem was well maintained via collateral flow from the PTA, and the patient demonstrated good recovery. A PTA may function as an anastomosis between the carotid and basilar systems, thus preventing a more serious infarction.