The aneurysm of an aortocoronary saphenous vein graft is a rare but potentially fatal complication of coronary artery bypass grafting. This case came to our observation after a single episode of hypotension, followed by dyspnea in a man previously operated on for coronary artery bypass grafting. A para-hilar mass was found on routine roentgenogram. The spiral computed tomographic scan was suggestive for aortic pseudoaneurysm. The correct diagnosis was obtained by cardiac catheterization showing a giant graft aneurysm determining compression and fistulous communication into the right atrium. The difficult diagnosis and the surgical treatment are discussed.