A 47-year-old male presented with a sudden onset of right hemiparesis and numbness of the left face. Magnetic resonance imaging demonstrated a lacunar infarct in the ventral pons. Cerebral angiography demonstrated a persistent trigeminal artery (PTA) anastomosing the left internal carotid artery to the distal basilar artery. Bilateral vertebral arteries and the basilar artery below the PTA junction were extremely hypoplastic. The bilateral posterior communicating arteries were embryonic. The posterior fossa circulation was almost independent from the circle of Willis. The poor vascular supply to the posterior fossa probably caused the brainstem infarct.