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Introduction: Acute brain injury incurred after aneurysm rupture in subarachnoid hemorrhage (SAH) is a major predictor of poor functional outcome. We hypothesize that platelet-leukocyte aggregates (PLA) form early after SAH and contribute to acute brain injury.Methods: A prospective study of antiplatelet-naive SAH patients and controls (patients with unruptured aneurysms undergoing repair) was conducted from 3/2014-3/2016. Platelet-monocyte, platelet-lymphocyte and platelet-neutrophil aggregates in whole blood were measured with and without exposure to a platelet agonist (Thrombin receptor activating peptide [TRAP]) using flow cytometry. PLA within 24h and averaged over 72h from ictus (prior to the onset of delayed cerebral ischemia/vasospasm) were compared between patients with mild (admission Hunt-Hess [HH] 1-3) versus severe early brain injury (EBI; HH 4-5).Results: We enrolled 60 SAH patients and 13 controls. PLA were significantly lower in those with severe EBI compared to those with mild EBI (Platelet-monocyte-aggregates 36% versus 53%, P=0.011; Platelet-neutrophil-aggregates 15.2 versus 23.1%, P=0.002) within 24h of ictus and prior to aneurysm repair and remained significantly lower over 72h (both P<0.05). Platelet-monocyte, platelet-neutrophil and platelet-lymphocyte aggregates were also significantly lower in those with severe EBI compared to controls (all P<0.05). The ability of platelets to be stimulated/activated by TRAP to form PLA was also lower in severe EBI patients compared to mild EBI and control patients over 72h (platelet-neutrophil-aggregates 79.7, 88.2 and 92.7%, respectively, P=0.003; platelet-lymphocyte aggregates 9.2, 11.0 and 14.6%, respectively, P=0.022), consistent with prior platelet activation/degranulation.Conclusions: PLA are lower, and respond less to stimulation in patients with severe EBI after SAH compared to those with mild EBI and controls. These data suggest that in severe EBI: PLA may form earlier and are cleared, are adherent to endothelium and not shed in the blood, or have migrated into the parenchyma. These hypotheses bear further study.