A 71-year-old man was admitted to our hospital with a diagnosis of subarachnoid hemorrhage (SAH). Angiographies revealed neither aneurysms nor vascular anomalies. However, these images elucidated the occlusion of the left cervical internal carotid artery as well as developed leptomeningeal anastomoses through the ipsilateral posterior cerebral artery, which resulted in blood perfusing the ipsilateral middle and anterior cerebral artery territories. Because the localization of SAH coincided with the developed leptomeningeal anastomosis, we speculated that the rupture of the developed leptomeningeal anastomosis in the basal cistern was the cause of SAH. We performed superficial temporal and middle cerebral artery bypass surgery to prevent rebleeding and ischemic stroke. In patients with occlusion of the internal carotid artery, SAH induced by the rupture of aneurysm formed by hemodynamic stress was recognized. However, rupture of developed leptomeningeal anastomosis should be considered as a possible cause of SAH of unknown origin. (Received March 7, 2016; Accepted August 31, 2016; Published January 1, 2017).