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Background and Objectives: Current evidence suggests that acute carotid artery stenting (CAS) for cervical lesions is associated with better functional outcomes in patients with acute stroke with tandem lesions (TLs) treated with endovascular therapy (EVT). However, the underlying causal pathophysiologic mechanism of this relationship compared with a non-CAS strategy remains unclear. We aimed to determine whether, and to what degree, reperfusion mediates the relationship between acute CAS and functional outcome in patients with TLs., Methods: This subanalysis stems from a multicenter retrospective cohort study across 16 stroke centers from January 2015 to December 2020. Patients with anterior circulation TLs who underwent EVT were included. Successful reperfusion was defined as a modified Thrombolysis in Cerebral Infarction scale ≥2B by the local team at each participating center. Mediation analysis was conducted to examine the potential causal pathway in which the relationship between acute CAS and functional outcome (90-day modified Rankin Scale) is mediated by successful reperfusion., Results: A total of 570 patients were included, with a median age (interquartile range) of 68 (59-76), among whom 180 (31.6%) were female. Among these patients, 354 (62.1%) underwent acute CAS and 244 (47.4%) had a favorable functional outcome. The remaining 216 (37.9%) patients were in the non-CAS group. The CAS group had significantly higher rates of successful reperfusion (91.2% vs 85.1%; p = 0.025) and favorable functional outcomes (52% vs 29%; p = 0.003) compared with the non-CAS group. Successful reperfusion was a strong predictor of functional outcome (adjusted common odds ratio [acOR] 4.88; 95% CI 2.91-8.17; p < 0.001). Successful reperfusion partially mediated the relationship between acute CAS and functional outcome, as acute CAS remained significantly associated with functional outcome after adjustment for successful reperfusion (acOR 1.89; 95% CI 1.27-2.83; p = 0.002). Successful reperfusion explained 25% (95% CI 3%-67%) of the relationship between acute CAS and functional outcome., Discussion: In patients with TL undergoing EVT, successful reperfusion predicted favorable functional outcomes when CAS was performed compared with non-CAS. A considerable proportion (25%) of the treatment effect of acute CAS on functional outcome was found to be mediated by improvement of successful reperfusion rates.