Aorto-coronary saphenous vein bypass graft implantation has led to a considerable relief of angina in a great majority of patients. Their functional capacity has been increased, and life possibly prolonged. During the postoperative period approximately ten per cent of the grafts occlude because of thrombosis, and later 2-3 per cent occlude per year. The angina returns in some patients, which is related either to graft closure or to progression of arteriosclerotic disease in the coronary arteries. A randomized clinical study has therefore been started in 142 patients in order to prevent graft closure. Two to three days after operation, one group of 72 receive acetylsalicylic acid, one gm. daily, and anticoagulants, while the 70 patients in the control group receive no antithrombotic drugs, and are asked to avoid anti-inflammatory agents. After 18 to 24 months, they are admitted for follow-up, including re-catheterization with selective angiography of coronary arteries or grafts. The effect of therapy will be assessed from graft potency alone.