Dissecting aortic aneurysm is a disease which is characterized by hemorrhagic intramural separation of aortic wall and extension for rating distances proximally, distally, or both from the site of the intimal tear. Most aortas show some type of medial degeneration most commonly described as cystic medial necrosis. DeBackey classified this disease according to involved aorta and site of intimal tear to 3 basic types, such as type I, II and III. Type III is defined that dissecting process arises in the descending thoracic aorta just distal to origin of the left subclavian artery and extends distally for a varing distance. We experienced a case of dissecting aneurysm, type III of DeBakey`s classification which dissecting process is limited to the descending thoracic aorta in the Department of Thoracic and Cardiovascular Surgery, Kyungpook National University Hospital. This patient was 40-year-old woman and she had suffered from intermittent sharp back pain for 3 years before admission. Excision of the aneurysm and Dacron graft were placed successfully under the left atrio-femoral bypass with artificial pump. The hospital course was uneventful.[KTCS 1981;3:291-296]