BACKGROUND AND PURPOSE: Endovascular thrombectomy is a standard treatment for acute ischemic stroke due to large-vessel occlusions (AIS-LVO), but a large minority of patients do not achieve successful reperfusion. This study aimed to investigate the hypoperfusion intensity ratio (HIR) as a prognostic biomarker in unsuccessfully reperfused patients with AIS-LVO. MATERIALS AND METHODS: A multicenter retrospective cohort study was conducted at 2 comprehensive stroke centers, involving patients with AIS-LVO who underwent endovascular thrombectomy but did not achieve successful reperfusion, defined as a modified TICI score of 0-2a. HIR, derived from CT or MR perfusion imaging, was analyzed for its association with favorable clinical outcomes (90-day mRs score of 0-2). The optimal HIR threshold predictive of favorable outcomes was identified through receiver operating curve analysis. RESULTS: Of 129 patients included, 20 (15.5%) achieved favorable outcomes. HIR of <
0.4 significantly predicted favorable outcomes with a sensitivity of 66% and specificity of 80%. Patients with an HIR of <
0.4 demonstrated better clinical and imaging profiles, including lower admission NIHSS scores and smaller ischemic core volumes. Multivariable logistic regression confirmed HIR, along with age and the presence of hemorrhagic transformation, as independent predictors of favorable outcomes. CONCLUSIONS: In unsuccessfully reperfused patients with AIS-LVO, an HIR of <
0.4 is associated with favorable outcomes, emphasizing on the importance of robust collateral circulation. This finding suggests that perfusion imaging and HIR evaluation could guide clinical decision-making and prognostication in this challenging patient subset.