Nontraumatic nonaneurysmal subarachnoid hemorrhage (SAH) is a rare condition. Among them, SAH with cerebral steno-occlusive disease is quite rare. Moreover there has been no report of SAH patient who had been diagnosed with steno-occlusive disease since before. We here report 2 cases of nontraumatic nonaneurysmal convexity SAH who originally had progressive cerebral steno-occlusive disease. Case 1, a woman in her 40s who had diagnosed left internal carotid artery (ICA) stenosis 6 years before complained of headache. She was diagnosed SAH and progressive ICA stenosis, then performed revascularization. Case 2, a woman in her 40s who had diagnosed right ICA stenosis 7 months before complained of headache. She was diagnosed with SAH and ICA occlusion. These 2 cases suggested that progressive cerebral steno-occlusive disease lead to SAH due to collapse of their fragile pial anastomoses.