Since the publication of preliminary results from the International Study of Unruptured Intracranial Aneurysms in 1998 there has been a great deal of debate concerning the natural history of these lesions and their attendant risk of aneurysmal subarachnoid hemorrhage. Therefore, the authors reviewed a selected number of crucial studies concerning this topic to determine the best evidence-based estimate of a rupture rate for these lesions. Based on this analysis, the yearly risk of bleeding for an unruptured intracranial aneurysm is estimated to be approximately 1% for aneurysms 7 to 10 mm in diameter. This risk of rupture increases with aneurysm size and it likewise diminishes as the size of the lesion decreases. This general rule serves as a reasonable interpretation of the results reported in the current body of literature.