Objective:To explore the association of cerebral venous outflow assessed by CT angiography (CTA) with first pass effect (FPE) in patients with acute anterior circulation large vessel occlusion accepted mechanical thrombectomy (MT).Methods:A retrospective analysis was performed; patients with acute anterior circulation large vessel occlusion accepted MT and CTA in Department of Neurology, Affiliated Hospital of Xuzhou Medical University from July 2018 to June 2021 were consecutively enrolled. Cerebral venous outflow in baseline CTA was evaluated using Cortical Vein Opacification Score (COVES). Patients were categorized into either FPE or non-FPE groups based on recanalization of occluded vessels after initial MT. General information, clinical features, radiological data, and surgery-related data between the 2 groups of patients were collected and compared. Significant variables ( P<0.1) from univariate analysis were included into a multivariable Logistic regression model to explore the relation between COVES and FPE. Predictive value of COVES in FPE was assessed using receiver operating characteristic (ROC) curve. Results:Out of the 143 patients enrolled in this study, 52 were into the FPE group and 91 were into the non-FPE group. Compared with the non-FPE group, the FPE group had higher COVES scores, higher proportion of patients with good cerebral venous drainage (COVES≥3), smaller core infarct volume, and shorter time from femoral artery puncture to vessel recanalization, with significant differences ( P<0.05). Multivariable Logistic regression analysis revealed that COVES was still corelated with FPE after adjusting covariates such as baseline NIHSS scores, core infarct volume, and time from femoral artery puncture to vessel recanalization ( OR=0.730, 95% CI: 0.567-0.940, P=0.015). ROC curve demonstrated that the combined model of COVES with aforementioned factors (COVES scores+baseline NIHSS scores+core infarct volume+time from femoral artery puncture to vessel recanalization) had an area under the curve of 0.757 (95% CI: 0.672-0.841, P<0.001), with sensitivity of 61.5% and specificity of 78.0%. Conclusion:Favorable cerebral venous drainage is an independent predictor for successful FPE in patients with acute anterior circulation large vessel occlusion accepted MT.