A 49-year-old man was referred for an episode of regressive hemiparesis, caused by proven brainstem infarction. Five weeks later, he started a series of posterior circulation transient ischemic attacks (TIAs) and angiography revealed left intracranial vertebral artery occlusion with contralateral severe vertebral artery stenosis and a significant collateral flow from the anterior spinal artery to the basilar artery. The TIAs stopped immediately after beginning of treatment with aspirin and heparin, and with this the patient has been asymptomatic for 4 years. This rare collateral pathway seems to be efficient in preventing hemodynamically induced ischemia in the posterior circulation, while antiaggregation and anticoagulation prevent further artery-to-artery embolism and may help to keep the collateral open.