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OBJECTIVEJapan has been reported to have the highest (and increasing) incidence of subarachnoid hemorrhage (SAH) in the world. However, there has never been a report on the nationwide incidence rate and recent trends for SAH in Japan. In this register-based study, the authors aimed to clarify the estimated nationwide SAH incidence rate and the recent trend in SAH incidence in Japan and the reasons for any changes in this trend.METHODSThe authors compiled data from the Japanese Ministry of Health, Labour and Welfare and from the records of the Japan Neurosurgical Society. They reviewed the age-standardized nationwide SAH mortality rate, the estimated age-standardized SAH incidence rate according to the age-standardized SAH mortality rate, and the estimated crude SAH incidence rate, including the 95% confidence intervals, from 2003 to 2015. The trends in the number of treatments for unruptured and ruptured cerebral aneurysms, as well as the prevalence of hypertension, current smoking status, and use of cholesterol-lowering drugs, were assessed. The estimated treatment rate for unruptured cerebral aneurysms (UCAs) was also calculated along with the 95% confidence interval.RESULTSThe estimated age-standardized nationwide SAH incidence rate significantly declined from 31.34 cases (95% CI 31.32–31.34) to 27.63 (95% CI 27.59–27.63; decrease by 11.8%) per 100,000 person-years. This decline was based on the 2010 population as the reference from 2003 through 2015 and a case-fatality rate of SAH that was assumed to decrease by 1% annually from 44% in 2003 to 32% in 2015. According to sensitivity analysis, the change rate of the estimated age-standardized SAH incidence rate ranged from −56.69% to 23.27%, with a mean of −30.91% (SD 20.52%), and there were decline trends in 92% of all trends. The estimated nationwide crude SAH incidence rate also showed a significant decline from 23.79 (95% CI 23.78–23.79) to 20.25 (95% CI 20.24–20.25; decrease by 14.9%) per 100,000 person-years. The trend in treatment numbers for UCAs increased significantly (p < 0.0001) from 2003 through 2015; however, the estimated treatment rate for UCAs was only 0.19 (95% CI 0.19–0.20) to 0.51 (95% CI 0.50–0.51) among all UCA patients. The prevalence of hypertension (males, p = 0.0003; females, p < 0.0001) and current smoking status (males, p < 0.0001; females, p = 0.0002) declined significantly from 2003 through 2015, while the use of cholesterol-lowering drugs increased significantly (males, p < 0.0001; females, p = 0.0005) during the same period.CONCLUSIONSThe estimated nationwide SAH incidence rate in Japan was higher than rates in other countries, although it has declined recently. An improving lifestyle may have contributed to the declining rate of SAH incidence in Japan.