Retrograde filling of seven anterior choroidal arteries from the posterior circulation was observed on a vertebral angiogram in six patients with occlusion of the proximal anterior choroidal artery or of the internal carotid artery. In one patient with thrombotic occlusion of the internal carotid artery, the enlarged anterior choroidal artery functioned as a major collateral pathway from the posterior circulation to the territory of the middle cerebral artery. Retrograde filling of the anterior choroidal artery is a sign of obstruction changes in the ipsilateral carotid artery at or proximal to the origin of the anterior choroidal artery.