Up to 90% of non-traumatic subarachnoid hemorrhage in North America is caused by a ruptured intracranial aneurysm. While any location in the circle of Willis may be the culprit, the majority are found arising from the anterior communicating artery, internal carotid.posterior communication bifurcation, middle cerebral artery bifurcation, and finally the basilar tip

Symptoms from a rupture are classically though of as "the worst headache of the patient's life". Most unruptured anuerysms may be clinically silent, although some can cause symptoms based on mass local mass effect such as cranial neuropathies. Less likely symtoms are ischemic - including transient ischemic attacks.

The risk of rebleed of a ruptured aneurysm is highest early on, and up to 50% of untreated ruptured aneurysm may rebleed within 6 months. Vasospasm is the leading cause of the death in ruptured aneurysm with peak incidence 3-21 days after rupture.