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A prospective study (SUAVe Study, Japan) is in progress at 13 national hospitals to determine the surgical indications for small unruptured intracranial aneurysm. These hospitals are observing the natural history of small unruptured aneurysms (under 5 mm diameter) without surgical treatment. Since October 2000, 455 aneurysms (393 patients) with 75 aneurysms excluded by film judgment committees have been registered, of which 380 aneurysms (329 cases) have been followed up for a mean of 13.8 months (5 aneurysms for 36 months, 68 for 24 months, 164 for 12 months, and 64 for 6 months). Four aneurysms have already ruptured (3 multiple and 1 single). The annual rupture rate was 0.8% (95% confidence interval 0.2-3%). Another 18 aneurysms have enlarged including seven aneurysms treated for enlargement over 2 mm in size. Considering this finding, the annual rupture rate might rise slightly. The important factors for rupture and enlargement were multiplicity of aneurysm, female, patient aged over 70 years old, and location of aneurysm (anterior communicating artery, basilar artery). The growth rate may be faster in multiple aneurysms than in single aneurysms.