Aortic dissection without entry and blood-flow in a false lumen was recently identified at necropsy and in vivo as intramural hemorrhage in the aortic wall (IMH). It was the purpose of the study to elucidate clinical signs and prognosis in this rare and poorly understood condition. Among 360 prospectively evaluated patients with clinical suspicion of aortic dissection, 195 patients presented with evidence of aortic disease. Of these, 25 patients (13.2%) had IMH of the thoracic aorta with no primary intimal tear, flap or overt dissection as shown by MRI (n = 12), contrast enhanced CT (n = 14) and TEE (n = 3). IMH was confirmed intraoperatively or at necropsy. There were 16 men and 9 women of ages 56 +/- 13 years; arterial hypertension was associated in 84% and Marfan's syndrome in 12%. IMH involved the ascending aorta in 12 (48%), the arch in 2 (8%), and the descending aorta in 11 cases (44%). IMH occupied 8.5 +/- 5 cm in length and 2.0 +/- 1.2 cm in aortic wall thickness. Both aortic regurgitation and pericardial/mediastinal effusion was present in 42% of type A and in 18% of type B IMH. IMH progression to overt dissection, rupture and/or acute tamponade occurred in 8/25 cases (32%) within 24 to 72 h, indicating the need for urgent surgical repair. The 30-day mortality of IMH inflicting the ascending aorta was 80% (4 of 5) with medical treatment in contrast to none (of 7) with early surgical treatment (p < 0.01). One-year survival was 71% in surgically treated patients and 20% with medical treatment (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)