This is a case report of 50 years old male suffering from pyrexia and epigastralgia 2 weeks prior to admission. On the 15th hospital day, he complained chest pain. Chest roentgenography revealed marked expansion of the mediastinum which was not existed upon admission. CT and MRI of the chest demonstrated a pseudoaneurysm of aortic arch at lesser curvature with extension into the pericardium. Exploration of the pericardium through median sternotomy disclosed fully filling with purulent effusion and blood clots. Following irrigation of the pericardial space and tube drainage, chest was closed. Repair of the aneurysm was postponded. Following continues irrigation of the pericardial space for 19 days, tube graft replacement of the aneurysm was performed under femorofemoral bypass and circuratory arrest through left thoracotomy. The aneurysmal space was filled with the omental pedicle to control infection. However, repeated bacteriological examination of the pericardium and aneurysm revealed no growth. Behçet disease was most suspected in report of the pathology.