Partial cardiopulmonary bypass, profound hypothermia, and circulatory arrest were used in the treatment of 25 patients with thoracic aortic aneurysms exposed through left posterolateral incisions. Indications included aortic clamp laceration (4 patients), pulmonary artery tear (1), treatment of ruptured aneurysm without clamping (5), right-sided arch (2), exposure of proximal aorta in a patient with a large aneurysm (1), inability to expose the proximal aorta for clamping (3), to permit removal of both arch and distal aorta (4), and to avoid distal arch clamping because of atheromatous disease (5). Aortic segments ranging from the upper descending thoracic aorta to most of the aorta were replaced, with early survival in 21 patients. Cerebral protection was satisfactory.