BACKGROUND: In acute aortic dissection, delays exist between presentation and diagnosis and, once diagnosed, definitive treatment. This study aimed to define the variables associated with these delays. METHODS AND RESULTS-: Acute aortic dissection patients enrolled in the International Registry of Acute Aortic Dissection (IRAD) between 1996 and January 2007 were evaluated for factors contributing to delays in presentation to diagnosis and in diagnosis to surgery. Multiple linear regression was performed to determine relative delay time ratios (DTRs) for individual correlates. The median time from arrival at the emergency department to diagnosis was 4.3 hours (quartile 1-3, 1.5-24 hours; n=894 patients) and from diagnosis to surgery was 4.3 hours (quartile 1-3, 2.4-24 hours; n=751). Delays in acute aortic dissection diagnosis occurred in female patients; those with atypical symptoms that were not abrupt or did not include chest, back, or any pain; patients with an absence of pulse deficit or hypotension; or those who initially presented to a nontertiary care hospital (all P<0.05). The largest relative DTRs were for fever (DTR=5.11; P<0.001) and transfer from nontertiary hospital (DTR=3.34; P<0.001). Delay in time from diagnosis to surgery was associated with a history of previous cardiac surgery, presentation without abrupt or any pain, and initial presentation to a nontertiary care hospital (all P<0.001). The strongest factors associated with operative delay were prolonged time from presentation to diagnosis (DTR=1.35; P<0.001), race other than white (DTR=2.25; P<0.001), and history of coronary artery bypass surgery (DTR=2.81; P<0.001). CONCLUSIONS-: Improved physician awareness of atypical presentations and prompt transport of acute aortic dissection patients could reduce crucial time variables.