This analysis indicates the results of a prospective study of 102 patients with 124 asymptomatic unruptured aneurysms without operation from 1993. The patients (M: 33, F: 69) were followed from 2 to 83 months (mean 38.2). The ages at diagnosis ranged from 21 to 78 years (mean 64.1) The locations were ICA: 48, MCA: 37, AcomA: 23, the vertebrobasilar artery: 9, ACA: 6 and PCA: 1. The average aneurysmal diameter was 4.3 mm, a range of 2 to 12 mm. They were followed up radiologically using MRA, MRI and 3D-CT angiography. Among the 102 patients, five had suffered subarachnoid hemorrhage (SAH) due to rupture of the aneurysms (MCA: 3, BA-SCA: 1, IC-PC: 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. The present study demonstrates that five asymptomatic unruptured aneurysms less than 10 mm in diameter subsequently ruptured. We ought to consider the data that unrup-tured aneurysms less than 10 mm in diameter have a very low probability of subsequent rupture published the New England Journal of Medicine (1998) very seriously.