A case of asymptomatic common carotid artery occlusion with direct vertebrocarotid anastomosis (proatlantal intersegmental artery) in a 70-year-old male is reported. Despite the acquired left common carotid artery occlusion, there was no demonstrable neurologic deficits that would occur by the involvement of the area supplied by the occluded carotid system. Cerebral angiography revealed, in addition to efficient collateral circulation from the contralateral carotid system through the anterior communicating artery, a collateral channel from the left vertebral artery to the ipsilateral external carotid artery via the proatlantal intersegmental artery. These collateral circulations were indicated to be important in prevention of possible grave neurologic consequences. The proatlantal intersegmental artery, though rarely visualized by ordinary cerebral angiographic procedures, was thus demonstrated to be capable of serving as a collateral channel of potential usefulness in the event of severe intracranial hemodynamic alterations, including acquired common carotid artery occlusion.