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Introduction: Various factors have been nominated for the rupture of unruptured cerebral aneurysms including size, shape and specific locations of aneurysms. Size ratio, which is the quotient of maximum size of the aneurysm divided by the diameter of the adjacent parent artery, has also been nominated as a strong indicator to discriminate ruptured versus unruptured cerebral aneurysms. However, this has never been tested as a risk in the prospective cohort. In this paper, we have assimilated size ratio in cases of UCAS Japan using reported average parent arterial size and evaluated its value.Methods: We used average main intracranial artery diameter reported in anatomical studies as follows: AComA: 1.5mm, MCA: 3.9mm, ICA: 4.3mm, PComA: 1.3mm, BA: 4.1mm and VA: 3.9mm. UCAS Japan cohort, which is a Japanese prospective cohort of unruptured cerebral aneurysms in Japan, were utilized (6,312 aneurysms with 11,011 aneurysm*year follow-up).Result: Rupture risks according to this assimilated size ratio (aSR) were as follows: aSR<1.0: 0.13%/yr, aSR:1∼1.5: 0.24%/yr, aSR 1.5∼2: 1.17%/yr, aSR: 2∼3: 1.03%/yr, aSR 3-; 2.62%/yr. With multivariate logistic regression model, aSR >1.5 was significant factor along with presence of daughter sac, symptomatic presentation and age >=70. Hazard ratio for the aSR>1.5 was 6.01 and the highest among these factors. Aneurysm locations lost its significance as the influencing factor.Conclusions: While our outcome was limited because the parent artery size was not measured in individual cases, our aSR showed clear demarcation of the future rupture risk. Rather than considering specific location as the risk factor, size ratio might be more important rupture predictor in assessing natural course of unruptured cerebral aneurysms.