Retrograde flow through the anterior spinal artery (ASA) from the cervical vertebral artery (VA) to the intracranial distal VA due to disrupted perfusion caused by bilateral VA occlusion is rare. We report two cases of hemodynamic vertebrobasilar circulatory insufficiency caused by bilateral VA occlusion. In these patients, the ASA filled in the retrograde direction, and provided collateral support to the ipsilateral posterior inferior cerebellar artery. The patients were treated with drip intravenous infusion of edaravone and/or argatroban. One patient had a good collateral supply from the posterior communicating artery and recovered almost completely within one month, but the other did not and lapsed into a coma, with generalized hyperreflexia, pin-point pupils, and ataxic respiration. Severe calcified lesions on three-dimensional computed tomography angiography at the occlusion site in the second patient indicated direct surgery including right superficial temporal artery to superior cerebellar artery anastomosis, rather than the endovascular approach. Retrograde flow through the ASA may be observed in this type of critical situation, and may be an important source of collateral supply to the posterior fossa territory.