Most are located on or near the circle of Willis. More than 90% are located at one of the following five sites: (a) the internal carotid artery at the level of the posterior communicating artery; (b) the junction of the anterior cerebral and anterior communicating arteries; (c) the proximal bifurcation of the middle cerebral artery; (d) the junction of the posterior cerebral and basilar arteries, and (e) the bifurcation of the carotid artery into the anterior cerebral and middle cerebral arteries.
--Rhoton AL. Neurosurgery 51[Suppl 1]:121–158, 2002
Most-common sites of saccular aneurysms.