A woman in her 70s first presented with asymptomatic right internal carotid artery (ICA) stenosis, which was discovered incidentally during investigation for dizziness, and was followed up with MRA for over 10 years. She experienced sudden onset of headache, and CT revealed SAH. Cerebral angiography revealed a plexiform arterial network from the C2 portion of the right ICA that extended from the sylvian fissure to the temporal lobe. Angiography was used to visualize the right middle cerebral artery (MCA) from the contralateral side via the anterior communicating artery. She was followed conservatively and discharged home with a modified Rankin scale score of 0. Three years after the onset of SAH, she has not experienced any rebleeding episodes. In this case, MRA revealed ICA stenosis, so antiplatelet drugs were administered, and she was followed up with MRA. However, MRA can only provide morphological evaluation with blood flow investigation and cannot allow detailed evaluation of nonvisualized blood vessels. Plexus vessels that develop as collateral circulation associated with asymptomatic intracranial ICA stenosis/occlusion can also be at risk of bleeding. Rather than continuing indecisive follow-up with MRAs, we recommend invasive tests such as cerebral angiography to accurately understand and follow any progressive development of this condition.