AbstractBackground and PurposeWe aimed to investigate the relationship between the degree and location of vertebrobasilar stenosis and quantitative magnetic resonance angiography (QMRA) distal flow.MethodsWe retrospectively reviewed patients who presented with acute ischemic stroke with ≥50% stenosis of the extracranial or intracranial vertebral or basilar arteries, and QMRA performed within 1 year of stroke. Standardized techniques were used to measure stenosis and to dichotomize vertebrobasilar distal flow status. Patients were grouped based on the involved artery and the severity of disease. All p‐values were calculated using chi‐squared analysis and Fisher exact test with statistical significance defined as p < .05.ResultsSixty‐nine patients met study inclusion, consisting of 31 with low distal flow and 38 with normal distal flow. The presence of severe stenosis or occlusion was 100% sensitive, but only 47% predictive and 26% specific of a low distal flow state. Bilateral vertebral disease was only 55% sensitive but was 71% predictive and 82% specific of a low‐flow state and was five times and nearly three times more likely to result in a low‐flow state compared to unilateral vertebral disease (14%) and isolated basilar disease (28%), respectively.ConclusionsSevere stenosis of ≥70% may mark the minimal threshold required to cause hemodynamic insufficiency in the posterior circulation, but nearly half of these patients may remain hemodynamically sufficient. Bilateral vertebral stenosis resulted in a fivefold increase in QMRA low distal flow status compared to unilateral vertebral disease. These results may have implications in the design of future treatment trials of intracranial atherosclerotic disease.