A 67-year-old man presenting with oppressive atypical chest pain 13 years after aorto-coronary saphenous vein bypass surgery, was found to have a large left anterior mediastinal mass on chest x-rays. Computerized tomography and digital subtraction angiography showed conflicting evidence on the vascular nature of the structure: the suspect diagnosis was of aortic pseudoaneurysm originating in the area of previous aortic cannulation. Selective coronary and bypass angiography showed extreme irregularity and dilatation of the grafts, and revealed the mediastinal mass to be represented by aneurysmal dilatation of the saphenous vein graft to the obtuse marginal coronary artery, partially filled with thrombus. At operation, another smaller aneurysm was found on the right coronary artery graft. Aneurysmal degeneration of vein grafts in the coronary circulation is exceedingly rare, and in only 6 reported cases has such an aneurysm presented as a mediastinal mass. To our knowledge, this is the first case in which an aneurysmal dilatation involved two saphenous vein coronary grafts.