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Background The efficacy of endovascular treatment (EVT) for acute vertebrobasilar artery occlusion (VBAO) with mild severity remains controversial. This study aimed to determine whether a threshold of stroke severity exists at which the efficacy of EVT changes in acute VBAO patients.Methods A retrospective cohort study including patients with acute VBAO from 65 stroke centers in China (December 2015 to June 2022) was conducted. The primary outcome was a 90-day modified Rankin Scale (mRS) score of 0–3. The interaction and visualization analyses were used to clarify the benefits of EVT changes with increasing National Institutes of Health Stroke Scale (NIHSS) score. Multivariable-adjusted logistic regressions were used to validate the intersection point of NIHSS score in the association between treatments and outcomes.Results Of 4000 acute VBAO patients, 2353 underwent EVT. A significant interaction effect of NIHSS score was observed in the association between treatments and outcomes (P for interaction <0.001), with an intersection point at 6.91. In patients with an NIHSS score <7, EVT combined with best medicine treatment (BMT) was associated with lower mRS 0–3 rates compared with BMT alone (adjusted OR (aOR) 0.63, P=0.031). In contrast, in the patients with an NIHSS score ≥7, the outcome was the opposite (aOR 2.24, P<0.001).Conclusions EVT should be cautiously considered in acute VBAO patients with an NIHSS score <7. Decisions require weighing baseline function, imaging features, and vascular anatomy. In the future, predictive models for EVT may enable more precise patient selection.