Based on the currently available evidence, we estimated cost-effectiveness of brain check-up with MRI/ MRA to diagnose and treat asymptomatic unruptured cerebral aneurysm.In this simulation analysis using a medical decision analysis model, we estimated costs and effectiveness under two different strategies ; one is asymptomatic 100, 000 Japanese population, aged mid-50's, receive brain check-up (Screen group), and another is nobody receive brain check-up (No Screen group). Costs included those for brain check-up (JPY 30, 000), diagnostic work-up (JPY 200, 000), treatment of unruptured aneurysm (JPY 2, 000, 000), treatment of ruptured aneurysm (JPY 4, 000, 000), and for long-term care ranging from JPY 1, 000, 000 to 5, 000, 000 per year according to the severity of disability. Effectiveness of brain check-up, in this analysis, was defined as the gain of life-years of survival, which is calculated as the difference of total life-years between the Screen group and the No Screen group.Based on the currently available evidence in Japan, we assumed the prevalence of asymptomatic unruptured aneurysm in the Japanese population aged mid-50's as 5%, sensitivity of brain check-up as 87%, its specificity as 92%. Likewise, we estimated the distribution of aneurysm size and the probability of spontaneous rupture according to the size, and the life and functional prognosis of the cases (see Text).The results indicated that mortality from subarachnoid hemorrhage would be decreased in the Screen group by about 80% than in the No Screen group. The cost for saving the life of one case with asymptomatic unruptured aneurysm was estimated to be JPY 74.4 × 106, and the cost for one life-year suvival was estimated to be JPY 2.4×106.