Abstract only Background: Laterality of the posterior cerebral artery (LPCA) in acute stroke has been related with a better leptomeningeal collateral circulation and with improved functional outcome at 6 months in patients treated with IV tPA. We aim to study the association between LPCA and the amount of brain tissue at risk of infarction in patients with anterior circulation arterial occlusion. Methods: From our prospective database of ischemic stroke we selected patients with anterior circulation arterial occlusion who underwent multimodal MRI < 12h of symptom onset. We considered LPCA when the following criteria were accomplished: a) ipsilateral PCA to the occlusion site was extended in 1 or more segments compared to the contralateral PCA and b) ipsilateral P4 segment was visible on axial TOF images. Two independent readers blinded to clinical data retrospectively assessed the presence of LPCA (k=0.65). We analyzed the association between LPCA and the volume of ischemic penumbra at baseline (Tmax>6s) and the final infarct volume (CT 24h). Good outcome was defined as mRS ≤ 2 at 90 days. Results: Seventy-two patients were included in the study (mean age 67y, 45% male). LPCA was present in 39 (54.1%). There were no differences between groups with or without LPCA, except a lower baseline NIHSS in the LPCA group (15 vs 19; p=0.003). Proportion of patients treated with reperfusion therapies was similar between groups. Patients with LPCA had a smaller lesion in Tmax>6s (54 vs 79cc; p=0.02), smaller final infarct volume (47 vs 111cc; p=0.013), and higher proportion of good outcome (52.8% vs 27.3%; p=0.03). In a multivariate analysis, LPCA was independently associated with smaller lesion volume on Tmax>6s (B -18, IC95% -36,-0.3), smaller final infarct volume (B -64.8, IC95% -100,-29) and better clinical outcome (OR 4.66, IC95% 1.04,20.8). Conclusion: LPCA sign in patients with anterior circulation arterial occlusion is associated with smaller volume of brain tissue at risk resulting in smaller infarct volume and better clinical outcome. These findings suggest favorable leptomeningeal collaterals.