Introduction: In Japan, asymptomatic unruptured cerebral aneurysms (UCA) are frequently discovered at brain check-ups. The overall annual rupture rate of these aneurysms has been reported as 0.95% according to UCAS Japan. Japanese people have a 2.8-times increased risk of rupture in UCA according to PHASES score. According to the recent reports, the rate of the small sized ruptured cerebral aneurysm ≤ 5mm (RCA) was over 35%. In this paper, we discuss the post hoc subgroup analysis of UCA in Japan special reference to small aneurysms. Subjects and Methods: From January 2001 to March 2004, a total of 6697 UCAs among 5720 patients were prospectively registered from any one of 283 institutions in Japan. In a subgroup of small UCAs ≤ 4mm (sUCA), aneurysms which developed a subarachnoid hemorrhage were extracted for this post hoc analysis. Intrapatient comparisons of each sUCAs were carried out using aneurysm specific factors such as size, location, calcification, thrombosis and shape, and patient specific factors such as age, sex, the reason of discovery for UCA, hypertension, hyperlipidemia, smoking, polycystic kidney and old cerebral infarction. Multivariate logistic regression analysis for rupture by JMP software was performed. Results: Eight sUCAs among 3132 sUCAs were ruptured and the annual rupture rate was 0.35% / year. By multivariate logistic regression analysis for rupture, the sCUA discovered by brain check-ups (OR: 3.69, CI: 1.02 – 15.5) and non-hyperlipidemia (OR: 5.65, CI: 1.11 – 103.9) were significant risk factors. The annual rupture rate of sUCA discovered by brain check-ups and with non-hyperlipidemia were 0.62 % and 0.42 %, respectively. Conclusions: SUCA has not indicated the category for radical treatment of UCA, however, the countermeasure for sUCA should be required to reduce the incidence of subarachnoid hemorrhage. Using of statin and any psychogenic stress might be related to rupture of sUCAs. Further precise research should be needed about the treatment for sUCA.