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Recent prospective studies have shown vertebrobasilar (VB) stenosis predicts stroke risk in posterior circulation stroke and transient ischemic attack. It is unclear whether this association is independent of other risk factors, and whether intracranial or extracranial stenosis confers different risks.A pooled individual patient analysis of data from 2 prospective studies was performed in 359 patients presenting with VB transient ischemic attack or stroke. Contrast-enhanced magnetic resonance angiography, or computed tomography angiogram, and clinical follow-up were available in 323 patients. Risk of stroke was calculated from any VB transient ischemic attack/stroke in the month before the presenting episode (first event) and from the presenting event. A systematic review of similar prospective studies was performed.Ninety-day risk of stroke from the first event was 24.6% in patients with VB stenosis versus 7.2% in those without (odds ratio, 4.2; 95% confidence interval, 2.1–8.6; P<0.0001). Risk was higher (33%) with intracranial (odds ratio, 6.5; 2.8–15.0; P<0.0001) than extracranial stenosis (16.2%; odds ratio, 2.5; 0.9–6.8; P=0.06). Risk from the presenting event was 9.6% in patients with stenosis versus 2.8% in those without (odds ratio, 3.7; 1.2–11.0; P=0.012), and again the risk was higher with intracranial stenosis. Cox regression showed the risk associated with VB stenosis was independent of other cardiovascular risk factors. The systematic review identified only 1 other report, which included only 6 patients.Symptomatic VB stenosis, particularly intracranial stenosis, is a strong independent predictor of stroke recurrence. The high early risk of stroke provides a strong rationale for randomized trials to determine whether stenting can reduce risk.