The pathological mechanisms of early brain injury (EBI) have remained obscure. Several studies have reported on the neuroradiological findings of EBI. However, to our knowledge, no study has attempted to explore the mechanism of EBI after subarachnoid hemorrhage (SAH). Therefore, this study evaluates whether the initial plasma D-dimer levels were associated with EBI, classifies magnetic resonance imaging (MRI) findings, and speculates about the mechanism of EBI. This study included 97 patients hospitalized within 24 h from the onset of nontraumatic SAH. The patients underwent MRI within 0-5 days from onset (before vasospasm) to detect EBI. EBI was radiologically defined as diffusion-weighted imaging (DWI)-positive lesions that appear dark on apparent diffusion coefficient maps, excluding procedure-related lesions. EBI, plasma D-dimer levels, and clinical features were retrospectively investigated. Elevated D-dimer levels were associated with poor outcomes. Patients with EBI had significantly higher D-dimer levels than those without EBI. EBI was detected in 24 patients (27.3%) of all, and in 22 (45%) of 49 patients with World Federation of Neurosurgical Societies (WFNS) grade 4-5 SAH. EBI was frequently observed in the paramedian frontal lobe. There were several types of the pathology in EBI, including widespread symmetrical cerebral cortex lesions, focal cortex lesions, periventricular injury, and other lesions impossible to classify due to unknown mechanisms such as thrombotic complication and microcirculatory disturbance, ultra-early spasm, and spreading depolarization. This study suggests that D-dimer levels predict poor outcomes in patients with SAH and that EBI was associated high D-dimer levels. Copyright © 2021. Published by Elsevier Inc.