There are cases of dissecting aortic aneurysm in which thrombotic formation occurs in the false lumen at an early stage, preventing dissection of the vessel wall and enlargement of the aneurysm. We studied such early thrombotic obliteration of false lumina in 12 (28.6%) of 42 patients with dissecting aortic aneurysm who underwent transesophageal two-dimensional echocardiography in the acute phase from June 1986 to October 1989. It was the first employment of a transesophageal approach at our hospital. In this study, we examined the clinical profiles of these patients as well as the usefulness of transesophageal two-dimensional echocardiography in establishing the diagnosis of dissecting aortic aneurysm and characterizing the disease. The patients were classified as Type I (2 patients) and Type III (10 patients; 6 Type IIIa and 4 Type IIIb) according to DeBakey's classification. The minimum and mean intervals from the onset of symptoms to transesophageal two-dimensional echocardiography were 1.5 hours and a mean of 38.2 hours, respectively. These results indicated that thrombotic obliteration of the false lumen had already occurred at a very early stage in some patients. The maximum diameter of the descending aorta was mean 37.6 mm with only 2 patients having that of 40 mm or more. Since left intrapleural hemorrhage was observed in these 2 patients (1 received emergent replacement of the descending aorta), enlargement of the aortic diameter may be indicative of unpredictable outcome, even in patients with early thrombotic obliteration of the false lumen. The long-term clinical course was favorable in all patients, including those who were treated surgically, over a mean follow-up period of 14.5 months.(ABSTRACT TRUNCATED AT 250 WORDS)