Endovascular treatment strategies to optimise individualised care for patients with vertebral artery (VA) stenosis need to be revisited. This study aimed to investigate the relationship between net VA flow volume (NVAFV) and the risk of posterior circulation infarction (PCI) in a high-risk patient population.We screened 1239 patients with extracranial VA stenosis, of whom 321 patients with severe VA V1 segment stenosis (≥70%) were enrolled in our study. We restratified the patients based on NVAFV and contralateral VA stenosis grades to analyse the proportion of each PCI mechanism-large artery atherosclerosis and branch artery occlusive disease. Furthermore, we estimated the incidence of recurrent ischaemic stroke between groups with different NVAFV over a follow-up period of 2 years.NVAFV was lower in the PCI group. Multiple logistic regression analysis showed that NVAFV is an independent risk factor for PCI and that the OR for PCI for the lowest NVAFV (<112.8 mL/min) was 4.19 (1.76 to 9.95, p=0.001). In patients with severe carotid artery disease, the OR for the lowest NVAFV was 14.03 (3.18 to 61.92, p<0.001). The lower NVAFV group had a higher incidence of recurrent ischaemic stroke events than the higher NVAFV group (HR 2.978, 95% CIs 1.414 to 6.272).Our study demonstrated that NVAFV, as estimated by colour duplex ultrasonography, was associated with the incidence of PCI and subsequent ischaemic events and that a high-risk population could be identified for further posterior circulation revascularisation.© Author(s) (or their employer(s)) 2021. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.