A case of persistent primitive proatlantal intersegmental artery (PPPIA) is reported. A 65-year-old male with treated hypertension was admitted to our clinic complaining of dysarthria and hemiparesis of sudden onset two days after the ictus. CT revealed spotty low-density lesions in the left corona radiata and bilateral thalami with bilateral watershed infarction. MRI findings were also compatible with cerebral infarction. Left common carotid angiography demonstrated a large anastomosis between the external carotid artery and the vertebral artery at the proatlantal region. Neither of the vertebral arteries were visualized on digital subtraction aortography. All the blood circulation of the vertebro-basilar system was through this anastomotic artery (PPPIA). A flow study revealed hypoperfusion in the territory of the left middle cerebral artery on 133Xe SPECT. Bone window CT of cervical vertebrae revealed hypoplasia of the left transverse foramen in C2, C3, C4, C5, C6 vertebrae. This case is very suggestive of an anaplasia or hypoplasia of the vertebral arteries. The etiology of his left frontal infarction seemed to be a blood-stealing phenomenon of long standing, from the anterior to the posterior circulation through the PPPIA.