A 47-year-old man presented with hemoptysis. From around June 20, 1995, he had symptoms of a common cold, but hemoptysis suddenly occurred on July 1 and he was brought to hospital. The chest X-ray film showed pleural effusion and enlargement of the mediastinum. CT scans showed calcification of the aorta, with a pseudoaneurysm 8 cm in diameter anterior to the descending aorta. When thoracotomy was performed under partial extracorporeal circulation, the thoracic cavity was found to be filled with bloody hydrothorax and hematoma, and the lower lobe of the left lung was adherent to the descending aorta. After a longitudinal incision was made in the aorta, a perforation 7 mm in diameter was identified in the posterior wall, and this was assumed to be the cause of the pseudoaneurysm. This part of the aorta was replaced with a Woven Dacron graft. Takayasu's arteritis was diagnosed by histological examination of the resected specimen, and the changes in the aortic wall at the site of perforation were considered to be chronic. About 30% of patients with this disease have aortic ectasia, but only a few pseudoaneurysms have been reported. Since the pseudoaneurysm occurred in our patient after reduction in the dose of medication, the development of perforation was suggested to be related to this change. In the future, this patient will require careful follow up for anastomotic aneurysm.