Background: Recent advances in the imaging techniques have identified both classic aortic dissection (AD) and intramural hematoma (IMH) as important entities of acute aortic syndrome (AAS), but data are limited regarding the clinical profiles and outcomes according to the different imaging diagnosis. Methods and Result: Clinical data of consecutive patients with AAS registered prospectively between 1994 and 2008 were reviewed. A total of 707 patients (mean age, 58.43.4 years; 54.6% male) were enrolled including 447 patients (63.2%) with AD and 260 (36.8%) with IMH. Type A AD was the most common (n=256, 36.2%), followed by type B AD (n=191, 27.0%), type B IMH (n=159, 22.5%), and type A IMH (n=101, 14.2%). Patients with IMH were older than those with AD (63.810.9 vs. 55.213.7 years, p<0.001). Patients with AD showed more frequent involvement of the ascending aorta (57.3% vs. 42.7%), whereas those with IMH showed higher frequency of distal aorta involvement (38.8% vs. 61.2%, p<0.001). Overall in-hospital mortality was 9.2% (65/707). Mortality rate of patients with type A AD was 17% and mortality rates of those with type A IMH, type B AD, and type B IMH were 8%, 5%, and 2%, respectively (p<0.001). Age, ascending aorta involvement, and classic AD were independent factors associated with in-hospital mortality, The 3-year cardiovascular mortality-free survival rates were 76.02.8% in type A AD, 84.24.0% in type A IMH, 85.62.8% in type B AD, and 92.92.3% in type B IMH, respectively (p<0.001). Conclusions: IMH comprises significant proportion of patients with AAS and shows different outcomes compared to classic AD. Besides the affected aorta site, underlying aortic pathology demonstrated by current imaging modalities is an independent prognostic factor in the clinical setting of AAS.