Background It is important to rapidly distinguish patients with acute aortic dissection of the ascending aorta (AADa) from those with acute myocardial infarction (AMI), because minimizing the time to initiation of reperfusion therapy leads to maximum benefits for AMI and erroneous reperfusion therapy for AADa can produce harmful outcomes. The aim of this study was to find a simple test to distinguish such patients. Methods and Results Data were collected from 29 consecutive patients with AADa and 49 consecutive patients with AMI who were admitted within 4 h of the onset of symptoms. The D-dimer concentration and the ratio of the maximum upper mediastinal diameter to the maximum thoracic diameter on plain chest radiograph (M-ratio) in the emergency room were studied retrospectively. Setting the cutoff values of the D-dimer concentration and the M-ratio to 0.8 or 0.9 μg/ml and 0.309, respectively, gave a sensitivity of 93.1% and 93.1% for AADa, respectively, and a sensitivity of 91.8% and 85.7% for AMI, respectively. Conclusions The D-dimer value and the M-ratio, with appropriate cutoff values, have potential as tests that can be routinely used to exclude AADa patients from patients diagnosed with AMI prior to reperfusion therapy. (Circ J 2005; 69: 677 - 682)