To evaluate the injury to ascending aorta by cross-clamp, the trauma to the vessel wall was examined microscopically and the occlusive intensity of the vascular clamp was measured. Six patients were underwent total replacement of the ascending aorta and the aortic arch for aortic dissection. The ascending aorta and the aortic arch were resected during operation, the clamped regions of the ascending aorta were cut into the four pieces (the distal site, two part of the medial site, the proximal site applied by the vascular clamp) after operation. The four pieces of the ascending aorta were examined microscopically. The occlusive intensity of the vascular clamp were measured at the distal site, the medial site, and the proximal site by the load-measuring scale. The trauma to the vessel wall were classified to three degrees (grade 1 to grade 3). Grade 1 showed the minimal changes--narrowing of interstices between collagen fibers and muscle fibers in the media. Grade 2 showed the moderate changes--decrease of collagen fibers, atrophy and disruption of smooth muscle and elastic fiber in the media. Grade 3 showed the severe changes--disruption of the intima and defluvium of endothelial cells. The occlusive intensity of the vascular clamp were 1.0 kg at the distal site, 1.9 kg at the medial site and 3.6 kg at the proximal site, respectively. In the microscopic examinations, grade 1 changes were most frequent at the distal site, grade 1 and grade 2 changes were observed equally at the medial site, grade 3 change was observed only at the proximal site. The degree of the trauma to the vessel wall becomes severer in proportion to increase of the occlusive intensity of the vascular clamp.