Nine patients with DeBakey type III aortic dissection (type III a; 6, type III b; 3) that underwent acute surgical therapy were studied with particular emphasis on the preoperative CT images and operative indication. It took 6 approximately 192 hours (mean 40.6 +/- 56.0) from onset of the disease to the operation. The reasons why operations were required in acute phase were prolonged pain; 2, fluid retensionin in thoracic cavity; 3, increasing fluid retension; 3, shock state; 1, anuria; 1.6 (75%) of 8 thoracic aortic dissection cases showed intra-thoracic fluid retension in chest roentogenogram or CT image. We performed prosthetic interposition of descending thoracic aorta in 8 patients and Y-graft interposition of abdominal aorta in a patient. During the operations, we found fluid retension in the thoracic and pericardial cavity in 83% (5 cases) of 6 non-ruptured cases. To diagnose ruptured aneurysm, CT image played very important role. But we could not discriminate between ruptured aneurysm and intrapleural serous fluid retension on the CT image. Puncture of intra-thoracic fluid is effective procedure to diagnose rupture of the aneurysms but it is impossible in some cases and traumatic tap may lead misdiagnosis. Therefore we think moderate or increasing fluid retension in the thoracic cavity should be added to the operative indication of acute DeBakey type III aortic dissection and that the early operation will make mortality rate lower.