Approximately 700,000 strokes occur annually in the U.S. making it the third leading cause of death and the leading cause of permanent disability among adults. Over one third of strokes occur in the posterior circulation, the leading cause of which is vertebrobasilar occlusive disease secondary to atherosclerosis. Symptomatic vertebrobasilar disease (VBD) carries a high annual risk of stroke, averaging 10-15% per year despite medical therapy. This represents a potentially treatable high risk stroke etiology. Advances in endovascular angioplasty and stenting have created new treatment options, but these interventions carry significant risks, and the selection criteria for appropriate candidates remains uncertain. Determining predictors of stroke in this population is the first step toward identifying those high risk patients most suitable for consideration of intervention. Our preliminary studies suggest that the risk of stroke in VBD is strongly related to the extent to which intracranial blood flow is compromised.