A 66-year-old woman, who had been suffering from frequent episodes of vertebro-basilar insufficiency for 10 years, was admitted because of brainstem ischemic symptoms. Magnetic resonance (MR) images were almost normal, but small flow voids were observed in front of the medulla oblongata and the upper cervical spinal cord. Cerebral angiography revealed that both of the vertebral arteries were completely occluded after branching off the respective posterior inferior cerebellar arteries. The basilar artery was fed only by a retrograde collateral flow via the anterior spinal artery which, arising from the left vertebral artery at the level of the fifth to sixth cervical vertebra, ascended the ventral aspect of the cord. These findings clarified that the flow voids on the MRI were from the anterior spinal artery, and were unusually prominent, as a collateral flow to the basilar artery. To the best of our knowledge, an anterior spinal flow void found on routine brain MR imaging has not been documented in the literature. Our case suggested that the flow void, a very small finding on the MRI, could be a useful sign in the diagnosis of serious vertebral arterial diseases.