Medium vessel occlusion (MeVO) contributes significantly to acute ischemic stroke (AIS). The hypoperfusion intensity ratio (HIR), reflecting collateral circulation via the ratio of Tmax >10s to Tmax >6s volumes, predicts infarct progression in large-vessel occlusions but is unstudied in MeVOs. In this multicenter, multinational retrospective study, we evaluated consecutive patients with MeVO who underwent mechanical thrombectomy with or without intravenous thrombolysis. Inclusion required available follow-up imaging and pretreatment CT perfusion. Follow-up infarct volume (FIV) was measured on CT or MRI 12-36 ​h post-procedure. Univariable and multivariable linear regression models were used to identify predictors of FIV, with HIR as the primary variable of interest. Among 147 patients (median age 75 years, 57 ​% female), univariable analysis showed HIR was significantly associated with larger FIV (β ​= ​80 ​mL; p ​< ​0.001). After adjusting for confounders, HIR remained independently associated with FIV (β ​= ​40 ​mL; p ​< ​0.001). Tmax >10 ​s showed the strongest correlation with FIV (r ​= ​0.56; p ​< ​0.001). These findings suggest that higher HIR correlates with larger infarct volumes, underscoring the prognostic role of collateral failure in MeVO and highlighting HIR as a potential imaging marker to guide treatment and outcome prediction.