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. Although a number of population-based studies have indi- cated that the incidence of aSAH has remained relatively stable over the past 4 decades,5,10 16 a recent review that adjusted for age and sex suggested a slight decrease in incidence between 1950 and 2005 for regions other than Japan, South and Central America, and Finland.4 These data are consistent with studies that show that the incidence of aSAH increases with age, with a typical average age of onset in adults 50 years of age.3,7,17,18 aSAH is relatively uncom- mon in children; incidence rates increase as children get older, with incidence ranging from 0.18 to 2.0 per 100 000.4,19 The majority of studies also indicate a higher incidence of aSAH in women than in men.7,11 13,20 22 Most recent pooled figures report the incidence in women to be 1.24 (95% confidence interval, 1.09 1.42) times higher than in men.4 This is lower than a previous estimate of 1.6 (95% confidence interval, 1.1 2.3) for the years 1960 to 1994.23 Evidence of a sex-age effect on aSAH incidence has emerged from pooled study data, with a higher incidence reported in younger men (25 45 years of age), women between 55 and 85 years of age, and men 85 years of age.4 Differences in incidence of aSAH by race and ethnicity appear to exist. 