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Background and Purpose: Hyperacute posterior circulation ischemia (PCI) is known to have poor clinical outcomes. Pretreatment ischemic lesion is an outcome predictor in patients with basilar artery occlusion. When basilar artery occlusion occurs, collateral blood flow can be reversed through hemodynamic connections between posterior communicating artery (PComA), first PCA segment, and carotid artery. We hypothesized that the presence of PComA is related to less severe ischemia and favorable outcome in the patients with hyperacute PCI.Methods: We retrospectively selected patients with acute ischemic lesion in posterior circulation who had received thrombolysis from stroke registry between June 2008 and September 2013. All patients were examined by CT angiography (CTA) before treatment. The NIHSS scale was used to assess initial stroke severity. The presence of PComA was confirmed based on axial CTA source images. The outcome of patients was defined as modified Rankin Scale score (mRS) after three months.Results: Seventy-five patients (45 male; mean age: 69 years, range; 29-91 years) were treated by multimodal thrombolytic therapy (27 intravenous only, 37 intra-arterial only, 11 intravenous and intra-arterial method). Thirty-four of 75 patients (45.3%) had bilateral PComAs and 36 of 75 patients (48%) had unilateral PComA. Initial NIHSS scale was significantly lower in patients with bilateral PComAs than patients with absent or unilateral PComA (mean NIHSS scale 12.38±9.02 vs. 18.73±9.19, p=0.004). The rate of favorable outcome (mRS 0-2) was significantly higher in patients with bilateral PComAs in comparison to patients with unilateral or absent PComA (78.6% vs. 27.3%, p<0.001).Conclusions: PComA is related to initial severity of hyperacute PCI. Well-developed collateral flow is associated with less severe ischemic damage, and as a consequence, leads to more favorable outcomes. Further prospective clinical trials are required to confirm the effect of PComA on clinical outcomes of hyperacute PCI.