A case of true venous aneurysm with saccular dilation of the proximal half of the superior vena cava, the right innominate vein, and the distal two thirds of the left innominate vein in an 18-year-old white woman is presented. At surgery part of the aneurysmal wall was resected, and subsequently the mediastinal venous system was reconstructed with use of the rest of the aneurysmal wall. The postoperative course was uneventful. Operative treatment of mediastinal venous aneurysms is indicated to prevent possible major complications. We strongly suggest performance of this surgery only by means of a heart-lung machine.