Patients with blockage of the blood vessels that supply blood to the back of the brain, known as vertebrobasilar disease (VBD), are at risk of having a stroke or temporary symptoms of a stroke known as transient ischemic attack (TIA). The risk of repeated stroke associated with VBD may be affected by several risk factors, including the degree to which the blockage reduces the blood flow to the brain. Patients with VBD have different levels of blockage ranging from partial blockage to complete blockage, which can affect the blood flow to the brain by variable amounts. The purpose of this research is to determine if patients with symptomatic VBD who demonstrate low blood flow to the back of the brain on magnetic resonance (MR)imaging are at higher risk of developing another stroke or TIA than patients with normal blood flow.