BACKGROUND AND PURPOSE: Urgent diagnosis of acute Stanford type A aortic dissection (AAD) in patients with acute ischemic stroke (AIS) or transient ischemic attack (TIA) is sometimes challenging. We assessed predictive values for markers of AAD in patients with AIS or TIA. METHODS: Consecutive patients with AIS or TIA with AAD who presented to our emergency room within 4.5 hours of symptom onset between 2007 and 2014 were compared with patients without AAD seen between 2012 and 2014. RESULTS: Data were obtained for 24 patients with AIS or TIA with AAD (15 women; mean age, 75 ± 12 years) and 812 patients without AAD (305 women; mean age, 73 ± 12 years). Compared with patients without AAD, patients with AAD displayed significantly higher systolic blood pressure (SBP) laterality (30 ± 20 mm Hg versus 12 ± 11 mm Hg), initial D-dimer concentration (median 38.1 µg/mL versus 1.3 µg/mL), and mediastinal width-to-chest width (M/C) ratio on x-ray (.35 ± .05 versus .29 ± .05), and more frequently showed common carotid artery (CCA) dissection on carotid ultrasonography (84% versus 1%) and pericardial effusion on echocardiography (43% versus 0%). Sensitivity and specificity to identify AAD were 80% and 75% for SBP laterality 17 mm Hg or greater; 100% and 86% for D-dimer concentration 4.1 µg/mL or greater; 75% and 76% for M/C ratio .32 or greater; 84% and 99% for CCA dissection; and 43% and 100% for pericardial effusion, respectively. CONCLUSIONS: High D-dimer level may provide the most reliable screening test for AAD in patients with AIS or TIA. CCA dissection on ultrasonography appears to represent the most disease-specific finding and shows acceptable sensitivity.