To develop a way to prevent the occlusion of the aortocoronary saphenous vein bypass grafts, we conducted a prospective, randomized, double-blind trial in 176 patients, some of whom received aspirin alone (975 mg/day) and some of whom took a combination of aspirin and dipyridamole (225 mg/day), and compared them with a parallel control group. Therapy was started 3 to 5 days after surgery. Follow-up angiography was performed in 142 patients (80%) at 1 year. Graft patency in the aspirin group (78%) was not significantly different from that in the aspirin-dipyridamole group (83%) or the control group (80%). Even in the patients at high risk for graft occlusion, i.e., those who had small recipient coronary arteries and poor graft flows, patency rates were not improved by the antiplatelet agents. When administered 3 to 5 days after surgery, aspirin alone or in combination with dipyridamole does not improve graft patency.