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Management of asymptomatic unruptured intracranial aneurysms remains controversial, and recent prospective follow-up studies showed that the rupture rate of small aneurysms is very low. These results are inconsistent with the finding that the majority of ruptured aneurysms in patients with subarachnoid hemorrhage are small.A Markov model was constructed to simulate the natural history of intracranial aneurysms. All epidemiological and statistical data obtained from the Portal Site of Official Statistics of Japan (e-Stat) were adjusted to the standardized age distribution. From the selected data of aneurysm formation, the prevalence of unruptured aneurysms was estimated as 1.45% and the incidence of subarachnoid hemorrhage calculated to be 19.7/100 000/year in the whole standardized population.The function for rupture rate constant with time was first analyzed. Selected values for annual rupture rates of 0.3%, 0.5%, 0.7%, and 1.0% showed inconsistencies in the relationship between the prevalence of unruptured aneurysm and the incidence of subarachnoid hemorrhage. Next, the function for a short period of high risk followed by a long period of low risk was considered. Annual rupture rates of 0.5%, 0.7%, and 1.0% indicated epidemiological compatibility with additional early rupture rates of 20%, 15%, and 10%, respectively.This study suggests that some aneurysms bleed shortly after formation and thus are rarely detected as unruptured aneurysms. Most aneurysms without early rupture remain stable for the remainder of life through some healing process, and prophylactic treatment for incidentally identified small unruptured aneurysms has no rationale.