This study defines the natural history of a selected group of 65 patients with 81 unruptured intracranial saccular aneurysms who did not undergo surgery. Eight of the 65 patients with aneurysms had intracranial hemorrhage due to aneurysmal rupture over a mean follow-up interval of slightly over eight years. We performed a multivariate discriminant analysis to assess the relation of several independent variables to aneurysmal ruptures. These variables included age, sex, aneurysm size, number of aneurysms, presence of multilobed aneurysms, aneurysmal symptoms other than rupture, aneurysm location, and hypertension at or before the identification of the aneurysm. The only variable of unquestionable significance was aneurysm size, which was noted in 73 instances. None of the 44 aneurysms smaller than 1 cm in diameter ruptured, whereas eight of the 29 aneurysms 1 cm or more in diameter eventually did. We conclude that unruptured intracranial saccular aneurysms smaller than 1 cm in diameter have a very low probability of subsequent rupture.