Abstract Body: Neutrophils are reported to be critical mediators of to poor outcome after subarachnoid hemorrhage (SAH). Following SAH, neutrophils cause vascular occlusion via neutrophil extracellular traps (NETs) and NETs have been identified as a therapeutic target to prevent delayed cerebral ischemia in mice (DCI) with SAH. In this study, our hypothesis was that markers of NETs are higher in aneurysmal SAH patients developing DCI compared to SAH patients not developing DCI. Moreover, we hypothesized NETs occlude blood vessels in the brain of SAH patients developing DCI. Using aneurysmal SAH patient blood collected at 1, 2, 4, 7, and 10 days post-SAH, we used ELISA kits to measure three markers of NETs: neutrophil elastase, citrullinated histone H3, and H3.1 nucleosome. We collected autopsy samples to examine for the presence of NETs in the brain vessels. Finally, as neutrophil counts (and the ratio of neutrophils to lymphocytes) are reported to be a biomarker for DCI, we assessed these measures too. Two hundred and sixty-nine aneurysmal SAH patients were included in our analyses: 193 patients did not develop DCI and 76 patients developed DCI. In our study, neutrophil elastase and citrullinated histone H3 were NETs markers that were able to distinguish between SAH patients who developed DCI and those who did not develop DCI. Specifically, neutrophil elastase was elevated in SAH patients developing DCI (compared to that of patients not developing DCI) and neutrophil elastase levels could predict which SAH patients were at high-risk for developing DCI as early as day 2 post-SAH. Citrullinated histone H3 displayed an elevated level in DCI patients vs no DCI patients on day 10. Moreover, NETs were observed occluding the brain blood vessels in SAH patients. None of the other measures had any predictive value for identifying which SAH patients were likely to develop DCI: neutrophil count, neutrophil to lymphocyte ratio, H3.1 nucleosome. The findings of this study suggest NETs factors may be useful biomarkers to predict which aneurysmal SAH patients are at-risk for DCI. As NETs were observed occluding brain vessels, NETs may also be a therapeutic target.