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A relatively frequent cause of subarachnoid hemorrhage is rupture of a saccular aneurysm. Most aneurysms occur at the bifurcation of large cerebral arteries. Over 90% of aneurysms occur in the circle of Willis and in the proximal middle cerebral artery. An aneurysm is caused by a focal weakening and thinning of the arterial wall. Rupture of an aneurysm can result in subarachnoid hemorrhage but it can also produce intracerebral hemorrhage and necrosis. Vasospasm and hydrocephalus (secondary to obstruction of arachnoid granulations) may be consequences of aneurysmal rupture. Rarely an unruptured aneurysm will reach a size capable of producing a mass effect on the brain.