We herein report a rare case of unruptured, giant left coronary sinus of Valsalva aneurysm and discuss surgical pitfalls associated with sinus of Valsalva aneurysms. A 63-year-old man was referred to us for clinical diagnosis and surgical treatment of a huge mass in the mediastinum. Enhanced computed tomography (CT) imaging revealed that the mass was a left coronary sinus of Valsalva aneurysm with a diameter of 74×57 mm ; moreover, the left coronary artery originated from the aneurysmal wall. In addition, echocardiography showed moderate aortic regurgitation (AR) caused by dilatation of the aortic annulus. Based on these findings, the Bentall procedure was selected for the Valsalva aneurysm and significant AR. The orifice of the aneurysm was 15×15 mm in size, and the aortic wall of the left coronary sinus was relatively thin. The left main trunk was injured due to severe adhesion between the trunk and the aneurysm ; therefore, vein patch repair was performed with a saphenous vein graft. Since the aortic annulus of the left coronary cusp was fragile, proximal anastomosis of the composite graft to the lesion had to be placed in the fibrous continuity between the aortic and mitral valves. With respect to the proximal anastomosis at the aortic annulus of the left coronary cusp, the suture line was covered with a bovine pericardium patch as there were no remnants of the normal aortic wall. The postoperative course was uneventful, and postoperative CT revealed complete resection of the aneurysm with no evidence of stenosis of the left main trunk.