The investigators recruited 4500-5000 native inhabitants aged 35-75 years lived in China according to China census-tract data from the year 2005. The gender was accordance with China census-tract data, and the population aged 35-75 years has a higher incident of intracranial aneurysm in the literature. Assuming an 8 percent prevalence of intracranial aneurysm in the general population and using a two-sided alpha value of 0.05, the investigators estimated that 4500-5000 native inhabitants would need to be enrolled for the study.