Dissecting aneurysm was described by Morgagni in 1761. Although the cystic medial necrosis has been thought as the major cause of dissecting aneurysm, diffuse atherosclerotic change of the aortic intima may contribute the medical degeneration. Recently, the authors experienced a case of dissecting aneurysm associated with diffuse atherosclerotic change in the intima, in an autopsy case. A 60 years old male patient was admitted to the hospital because of indigestion and chilling sensation for 7 days. Three days prior to this entry, he experienced a sudden crushing chest pain, that was spontaneously releaved several hours later. He denied any past history of coronary heart disease, valvular hear disease or syphilitic infection. On admission, physical examinations revealed slightly puffy face, distended jugular veins, and palpable liver. Chest x0ray film revealed elongation and distortion of the greater vessels accompanied by cardiomegaly. Laboratory findings including complete blood count, blood chemistry, urinalysis, and VDRL were within normal limits. During admission, he was observed with supportive therapy. On the 8th hospital day, 18 days after first symptoms developed, he suddenly fell into unconscious state during defecation, and died soon after. The autopsy was limited to the cardiovascular system. The heart weighed 670gm, and about 1,500 cc of blood and clots were noted in the pericardial cavity. There was a rupture, 2.5×1.7㎝, in the ascending aorta near the aortic value, on the point 5㎝ distal from the aortic valve. The wall of the aorta was dissected near the aortic valve to the lower abdominal aorta (De Bakey Type I). Diffuse atherosclerotic change of the aortic intima was observed, but atheromatous ulcer was not observed. Microscopic findings revealed diffuse hyalin degeneration of the aortic media, intimal thickening and subintimal aggregations of foam cells with focal calcification.