Aspirin (325 and 900 mg/d) is effective for a period of 1 year in reducing the frequency of saphenous vein bypass graft occlusion when begun 1 day before operation or on the day of operation. Aspirin in combination with dipyridamole is not more effective than aspirin alone in the prevention of saphenous vein graft occlusion. Bleeding is higher among patients treated with aspirin (325 mg/d) than among controls if aspirin is started 1 day before operation. Bleeding in one trial was greater than controls if aspirin (300 mg/d) was started the day of operation, and in one trial there was no difference when aspirin (325 mg/d) was started the day of operation. Ticlopidine (500 mg/d), started 2 days after operation, was effective in maintaining graft patency. Oral anticoagulants were inconsistent in the maintenance of saphenous vein graft patency. The continued use of aspirin for 2 additional years after an initial year of aspirin therapy for the prevention of saphenous vein bypass graft occlusion showed no additional long-term benefit on graft patency at the end of the third year. Antithrombotic agents given to patients with internal mammary artery bypass grafts showed no benefit in comparison to placebo because patency on placebo was high.