BACKGROUND AND OBJECTIVES: Although aortic intramural hemorrhage (AIH) is different from classic aortic dissection (AD) in terms of absence of continuous direct flow communication through intimal tear, the same treatment strategy, emergent surgical repair, has been applied for patients with AIH involving the ascending aorta. The impact of different false lumen hemodynamic has not been seriously investigated and clinical features of AIH and AD have not been directly compared. METHODS: From 1990 to December 1998, clinical evaluation with various imaging modalities confirmed the diagnosis of proximal AD and AIH in 73 and 18 patients, respectively. Direct comparison of clinical data including clinical features, hospital course, and follow-up data was performed retrospectively. RESULTS: Patients with AIH were older (69+/-10 years-old 49+/-14, p<0.05) and female was predominant in AIH (15/18 vs. 26/73, p<0.05). The development of mediastinal hemorrhage, pericardial and pleural effusion was more frequent in AIH than in AD. In-hospital mortality was same in both groups (11% in AIH vs. 17% in AD, p=NS). Although medical treatment was more frequently selected in AIH group (61% vs. 12%, p<0.05) due to old age and other associated medical diseases, mortality rate with medical treatment was much lower in AIH than in AD (9% vs. 66%, p<0.05). Among 11 patients with AIH, in whom medical treatment was chosen, 10 patients were discharged without any event. In follow-up imaging studies of 8 survived AIH patients without surgical repair, 4 patients showed complete resolution. Typical AD developed in 2 about 2 months after the acute event, and the other 2 patients showed focal AD only in the descending aorta. The 3-year survival rate of AIH group was 89+/-7%, which was not significantly different from that of AD group (75+/-6%, p>0.05). CONCLUSIONS: Patients with proximal AIH shows different clinical features and much better prognosis with medical treatment compared to those with AD. These results support our initial hypothesis that AIH is not just a precursor of overt AD but a distinct disease entity and absence of continuous flow communication in AIH can have different clinical impact. This should encourage systematic investigations to find out the predictors of development of complications and to assess the role of elective surgery with frequent imaging follow-up in proximal AIH.