AbstractWe aimed to study the relationship between the ischemic core’s (IC) radiological hypodensity and the risk of parenchymal haematoma after endovascular therapy (EVT) in acute ischemic stroke (AIS) presenting > 4.5 h from onset. We studied AIS patients with a proximal anterior circulation occlusion > 4.5 h from symptoms onset treated with primary EVT. The IC regions of interest (ROI) were manually delineated on pretreatment CT within the affected hemisphere and their specular ROIs on the unaffected side. IC hypodensity ratio was calculated by dividing mean Hounsfield Unit (HU) value from all ROIs in affected/unaffected hemisphere. Primary endpoint: parenchymal hematoma (PH) type hemorrhagic transformation. Secondary: poor long-term clinical outcome. From May 2015 to November 2018, 648 consecutive AIS patients received reperfusion therapies and 107 met all inclusion criteria. PH after EVT was diagnosed in 33 (31%) patients. In bivariate analyses, IC hypodensity ratio (p < 0.001) and minimum HU value (p = 0.008) were associated with PH. A lower IC hypodensity ratio [OR < 0.001 (< 0.001–0.116) p 0.016] predicted PH but not poor clinical outcome in multivariable logistic regression models. A lower IC radiological density predicted a higher risk of PH in > 4.5 h-window AIS patients treated with primary EVT, although it was not independently associated with a worse clinical outcome.