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We previously derived the Ottawa Subarachnoid Hemorrhage Rule to identify subarachnoid hemorrhage (SAH) in patients with acute headache. Our objective was to validate the rule in a new cohort of consecutive patients who visited an emergency department.We conducted a multicentre prospective cohort study at 6 university-affiliated tertiary-care hospital emergency departments in Canada from January 2010 to January 2014. We included alert, neurologically intact adult patients with a headache peaking within 1 hour of onset. Treating physicians in the emergency department explicitly scored the rule before investigations were started. We defined subarachnoid hemorrhage as detection of any of the following: subarachnoid blood visible upon computed tomography of the head (from the final report by the local radiologist); xanthochromia in the cerebrospinal fluid (by visual inspection); or the presence of erythrocytes (> 1 × 106/L) in the final tube of cerebrospinal fluid, with an aneurysm or arteriovenous malformation visible upon cerebral angiography. We calculated sensitivity and specificity of the Ottawa SAH Rule for detecting or ruling out subarachnoid hemorrhage.Treating physicians enrolled 1153 of 1743 (66.2%) potentially eligible patients, including 67 with subarachnoid hemorrhage. The Ottawa SAH Rule had 100% sensitivity (95% confidence interval [CI] 94.6%–100%) with a specificity of 13.6% (95% CI 13.1%–15.8%), whereas neuroimaging rates remained similar (about 87%).We found that the Ottawa SAH Rule was sensitive for identifying subarachnoid hemorrhage in otherwise alert and neurologically intact patients. We believe that the Ottawa SAH Rule can be used to rule out this serious diagnosis, thereby decreasing the number of cases missed while constraining rates of neuroimaging.