Purpose: Acute myocardial infarction (AMI) concomitant with acute aortic syndrome (AAS) is rare but prompt recognition of concomitant AAS is critical, particularly in patients with ST-segment elevation myocardial infarction (STEMI) because misdiagnosis with early thrombolytic or anticoagulant treatment may result in catastrophic consequences. This study examined the clinical features of patients of STEMI concomitant with AAS that may be a diagnostic clue. Methods: Between January 1, 2010 and December 31, 2014, 22 patients who had the initial diagnosis of acute coronary syndrome (AMI and unstable angina) and AAS (aortic dissection, intramural hematoma, and ruptured thoracic aneurysm) in our emergency department were reviewed. Among them, 10 patients who were transferred from other hospitals and 4 patients with non-STEMI were excluded, leaving 8 patients of STEMI concomitant with AAS for analysis. Results: The mean age of study patients was 57.5±16.31 years and five patients were Stanford type A and three patients were type B aortic dissection. Six patients had STsegment elevation in anterior leads and 2 patients in inferior leads. Most patients had acute onset and severe chest pain, but none had dissecting nature chest pain. Serum troponin I was elevated in three patients but all patients had Ddimer elevation. Aortic regurgitation or regional wall motion abnormality was detected in four patients, and widened mediastinum was observed in all study patients. Conclusion: Concomitant AAS might be suspected in patients with STEMI who have elevated D-dimer and widened mediastinum.