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Background: Collateral circulation is a key factor in the pathophysiology of ischemic stroke. We conducted detailed analyses of angiography acquired immediately prior to endovascular therapy in the HERMES collaboration of recent landmark thrombectomy trials to determine predictors of collateral status and assess impact on clinical outcomes.Methods: The HERMES Imaging Core, blind to all other clinical and imaging data, independently interpreted conventional angiography acquired immediately prior to endovascular therapy. Collaterals were graded with the ASITN scale, based on available data for the site of arterial occlusion defined on initial injections. The statistical core analyzed the association of collateral grade with demographics, baseline NIHSS, site of arterial occlusion and clinical outcomes of day 90 mRS.Results: Angiography of collaterals was available in 376/605 (62%), including ASITN grades 0 in 7 (2%), 1 in 40 (10%), 2 in 182 (48%), 3 in 129 (34%) and 4 in 18 (5%). Elevated blood glucose (p=0.011) and diabetes (p=0.048) were associated with worse collateral grades, but age and NIHSS were unrelated. Better collateral grade was strongly associated with the degree of subsequent TICI reperfusion (p<0.001). The limited numbers of symptomatic intracranial hemorrhage (14/376 or 3.7%) or parenchymal hematomas (18/376 or 4.8%) precluded analysis of association with collateral status. Collaterals had strong impact on mRS shift from baseline to 90 days (p<0.001).Multivariable regression revealed that better collateral status was a potent determinant (OR 1.37 per grade, p=0.028) of outcomes, equivalent to 4.5 points of NIHSS and 14 years of age in terms of impact on mRS outcomes.Conclusions: Collaterals at angiography were a potent determinant of clinical outcomes in recent landmark thrombectomy trials.