PURPOSE: The purpose of this study was to identify common underlying etiologies that may be responsible for isolated acute nontraumatic cortical subarachnoid hemorrhage (cSAH) by analysis of magnetic resonance imaging (MRI) findings of the brain. MATERIALS AND METHODS: From August 2005 to February 2014, 15 cSAH patients were admitted to our institution. All patients with cSAH underwent brain MRI and magnetic resonance angiography as a part of their initial evaluation. An analysis of the patients' medical history, clinical presentations, and brain MRI findings was retrospectively performed. RESULTS: Among the combined pathologies that were suspected causes of cSAH, 11 patients showed acute or subacute cerebral infarctions at the ipsilateral hemisphere of cSAH on the diffusion-weighted images. Four of 11 patients had only cerebral infarction, but the other 7 had combined vasculopathy of extra- and intracranial arteries. Four of 15 patients who did not have cerebral infarction, had intracranial artery stenosis, or showed possible cerebral amyloid angiopathy, or no abnormal findings on the brain MRI. CONCLUSION: Ischemic stroke, such as cerebral infarction or vasculopathy of the extraand intracranial arteries is regarded as a common underlying etiology of the cSAH based on MRI findings.