The natural history of asymptomatic unruptured aneurysms is not clear. We conducted a follow up study of 100 patients (since 1993) with 122 asymptomatic unruptured aneurysms that had not been operated on. We report five patients with previously documented asymptomatic unruptured aneurysms smaller than 10 mm in diameter that subsequently ruptured. Among the 100 patients, five had suffered subarachnoid hemorrhage (SAH) due to rupturing of an aneurysm. Of the 5 cases, 1 was male and 4 were female, with ages ranging from 59-73 years (mean age, 68 years). The aneurysms were on the MCA in 3, on the BA-SCA in 1, on the IC-PC in 1. The maximal diameter of the aneurysms at diagnosis ranged from 4.5 to 8 mm. The period from discovery to SAH was from 4 to 69 months and the cumulative rate of rupture of the aneurysms was 1.5 percent per year. Four of the 5 cases increased in size after the rupture. In our series, 2 of the 5 cases showed enlargement and the development of an aneurysmal bleb in the follow up MRA and 3D-CTA. The present study demonstrates that five asymptomatic unruptured aneurysms less than 10 mm in diameter subsequently ruptured. We ought to seriously consider the assertion published in the New England Journal of Medicine (Dec. 10, 1998), that unruptured aneurysms less than 10 mm in diameter have a very low probability of subsequent rupture.