IntroductionFirst-pass(FP)-recanalization has been shown to improve outcome in patients with ischemic stroke undergoing mechanical thrombectomy(MT). Data also suggests that FP-recanalization is more often associated with complete reperfusion TICI=3 than with TICI=2b. Independently, it was shown that TICI=3 significantly improves functional outcome after mechanical thrombectomyAim of StudyTo evaluate whether early recanalization or complete recanalization TICI=3 are the determinants of improved outcome observed after FP-recanalization.MethodsAll patients prospectively enrolled in the German Stroke Registry-ET (05/2015–12/2021;N=13082) were screened. Inclusion criteria were anterior circulation stroke and successful recanalization TICI≥2b. Good functional outcome was defined as 90d modified Rankin Scale(mRS)≤2. Mediation analysis was performed to evaluate how much of the FP-related improvement in functional outcome is explained by complete reperfusion TICI=3.Results2589 patients were included, 1170(47%) had successful FP recanalization, 797(68% of FP-cases) with TICI=3. FP-recanalization was associated with higher rate of good functional outcome compared to multi-pass with 49.2% vs 37.6%. Mediation analysis suggests that FP-recanalization increases probability of good outcome by 9.6 percentage points vs. multi-pass recanalization. 12.8% (95%CI:7.6%-23%) of this effect was explained by TICI=3 recanalization whereas 87.2% (77%-92%) are explained by other factors associated with FP- recanalization.ConclusionOnly 13% of the FP-related improvement in functional outcome is explained by higher rates of complete recanalization, suggesting significant importance of early recanalization and low number of MT maneuvers. Results may improve the understanding of the importance of FP-reperfusion vs. early TICI=3 and may help to optimize MT treatment strategies.Disclosure of InterestHK has financial interest in Eppdata GmbH.GT received fees as consultant and lecturer from Acandis, Alexion, Amarin, Boehringer Ingelheim, Bayer, BMS/Pfizer, Daiichii Sankyo and Portola. He serves in the board of the TEA Stroke Study and of ESO.JF is consultant for Cerenovus, Medtronic, Microvention, Penumbra, Phenox, Roche, Stryker and Tonbridge. He is stock holder of Tegus Medical, Eppdata and Vastrax. He serves as Associate Editor at JNIS.All other authors have nothing to disclose.