OBJECTIVE: This study investigated the proportion of small cerebral aneurysm (<5 mm) to help in the decision making concerning unruptured small aneurysms using the proportion of ruptured small aneurysms as a guide. METHODS: The records of aneurysm patients treated in our hospital from January 2004 to December 2006 were retrospectively reviewed. Patients with ruptured aneurysms were divided into five groups according to their sizes (Group1 defined as tiny :< 3, Group2 defined as very small: 3~5, Group3 defined as small: 5~7, Group4 defined as medium: 7~10 and Group5 defined as large: > 10mm). The clinical and radiological findings of the ruptured aneurysms were also evaluated. RESULTS: The mean age of the 244 enrolled patients was 54.6 years. The mean size of the rupture and the unruptured aneurysms was 6.8 mm. The proportions of aneurysm sizes were 7.8% (<3 mm), 30.7% (3~5 mm), 25.4% (5~7 mm), 19.3% (7~10 mm) and 16.8% (<10 mm). CONCLUSION: The proportion of small ruptured aneurysms (<5 mm) was appreciable; the proportion was not small just to observe. The proportion of very small ruptured aneurysms (3~5 mm) was significant in patients who required surgery or endovascular coiling. Although tiny aneurysms (<3 mm) may have a very low risk of rupture, they should be closely followed-up to preclude such a catastrophe.