Incidence and Prevalence of aSAH Considerable variation in the annual incidence of aSAH exists in different regions of the world. A World Health Organization study found a 10-fold variation in the age- adjusted annual incidence in countries in Europe and Asia, from 2.0 cases per 100 000 population in China to 22.5 cases per 100 000 in Finland.3 A later systematic review supported a high incidence of aSAH in Finland and Japan, a low incidence in South and Central America, and an intermediate incidence of 9.1 per 100 000 population in other regions.4 In a more recent systematic review of population-based studies, the incidence of aSAH ranged from 2 to 16 per 100 000.5 In that review, the pooled age-adjusted incidence rate of aSAH in low- to middle-income countries was found to be almost double that of high-income countries.5 Although some reports have suggested the incidence of aSAH in the United States to be 9.7 per 100 000,6 the 2003 Nationwide Inpatient Sample provided an annual estimate of 14.5 discharges for aSAH per 100 000 adults.7 Because death resulting from aSAH often occurs before hospital admission (an estimated 12% to 15% of cases),8,9 the true incidence of aSAH might be even higher. 