A successful nearly total aortic replacement for DeBakey type-I aortic dissection in a patient with Marfan's syndrome is reported. A 36-year-old woman developed a DeBakey type-I aortic dissection in October 1987. We replaced the ascending aorta and resuspended the aortic valve urgently, when she had not been diagnosed with Marfan's syndrome. In August 1989, we replaced the distal portion of the descending aorta and the infra-renal abdominal aorta because of dilatation in the diameter of these regions. In September 1990, she had sudden severe chest pain. Computed tomography and Doppler ultrasonography revealed that a new aortic dissection developed in the aortic arch and the descending aorta (three-channeled dissection) and that the new false lumen compressed the true and the old false lumens obstructing the blood flow to the abdominal aorta. She was immediately placed an axilo-femoral bypass, and in January 1991 she underwent the arch and descending aorta replacement using permanent bypass to the supra-aortic branches and partial extracorporeal circulation (first reported by Larmi et al). Postoperative course was satisfactory, and she has been well at 3 years after the operation. Because progressive dilatation of an untreated dissecting aorta is common in patients with Marfan's syndrome, aggressive replacement of the diseased aorta is indicated in these patients.