Background and Purpose: Acute Stanford type A aortic dissection (AAD) is a devastating aortic disease, and prompt diagnosis is sometimes difficult to make. Identification of AAD in suspected acute stroke patients is especially challenging. Nevertheless, the frequencies and predictive factors of AAD in suspected acute stroke patients have not been well investigated. The aim of this study was to elucidate the prevalence of and predictors for AAD in patients with suspected acute stroke. Methods: From January 2012 through January 2013, consecutive patients who visited our emergency department (ED) due to suspected acute (<24 h from onset) stroke were retrospectively enrolled. Clinical parameters including systolic blood pressure (SBP) and laboratory data were collected. Frequency of AAD in suspected acute stroke patients and acute ischemic stroke (AIS) subjects were assessed, and factors associated with AAD among AIS patients were investigated. Results: A total of 1,637 patients were included in this study. Five patients (0.31%, 95% CI 0.04-0.57) were diagnosed as having AAD. The prevalence of AAD in all AIS individuals during the study period was 1.09% (95% CI 0.14-2.05), and AAD accounted for 1.70% (95% CI 0.05-3.36) of AIS patients who appeared at the hospital within 4 h from onset. Most AAD patients presented with disturbed consciousness, and none of the AAD patients complained of chest pain. Neck ultrasonography detected an intimal flap in AAD patients. Two AAD cases died soon after ED arrival. The remaining 3 were promptly diagnosed as having AAD in the ED and underwent emergency surgery; all were discharged with only mild neurological symptoms. Low SBP in the right arm (cut-off value ≤110 mm Hg, sensitivity 100%, specificity 94.4%) and high D-dimer level (cut-off value ≥5.0 μg/ml, sensitivity 100%, specificity 91.7%) had high predictive values for detecting AAD in patients with AIS presenting within 4 h from onset. Conclusions: AAD was seen in 0.31% of suspected acute stroke patients and 1.70% of AIS patients presenting within 4 h from onset. AAD patients who were initially suspected as having acute stroke had severe neurological symptoms, including disturbance of consciousness, did not complain of typical chest pain, and when emergency surgery was performed, favorable neurological and survival outcomes were achieved. Low SBP in the right arm and high D-dimer level could predict AAD.