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Ten patients with solitary stenosis of the left main coronary artery underwent coronary artery bypass grafting. All patients suffered from unstable angina and were in NYHA class III or IV. Two of them required intravenous infusion of nitroglycerin preoperatively. The degree of stenosis of the left main coronary artery was 75% in 3 patients, 90% in 5, 95% in one and total obstruction in the other one. Five patients received saphenous vein grafts to the LAD and circumflex artery and the other 5 patients received IMA grafts to the LAD and saphenous vein grafts to the circumflex arteries. In 2 patients with 75% stenosis of the left main coronary artery we found narrow internal mammary artery grafts, the so called "string sign", on postoperative angiography. Although several causes of string sign were proposed previously, we supposed that the main cause of the "string sign" was the competition for flow between the IMA graft and the native coronary artery or grafted coronary artery. Postoperatively, all patients showed improvements in cardiac function and were in NYHA class I. No evidence of ischemic findings was found in postoperative exercise stress tests.