The objective is to conduct a longitudinal study of patients with symptomatic VBD. Our central hypothesis is that patients with symptomatic VBD who demonstrate limitation of blood flow on quantitative magnetic resonance angiography (QMRA) are at higher risk of stroke.The primary aim of this proposal is to test the hypothesis that among patients with VBD, those with distal blood flow compromise are at higher risk of subsequent posterior circulation stroke than those with normal flow.