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OBJECTIVE To determine whether slow-release enteric coated aspirin (100 mg daily), commenced before operation, improves the patency of saphenous vein (SV) coronary artery bypass grafts at six months. DESIGN AND SETTING Double-blind, randomised, placebo-controlled study at a teaching hospital. RESULTS One hundred and forty patients were randomly allocated to receive enteric coated aspirin or matching placebo. Similar groups of 50 (aspirin) and 52 (placebo) subjects completed the six months follow-up and had an angiogram to assess patency. Five patients treated with aspirin and nine who received placebo had at least one occluded SV graft; the distal ends of 6 of 128 SV grafts in aspirin-treated patients (4.7%) and 13 of 145 SV grafts in patients in the placebo group (9.0%) were occluded--the difference was not significant. An arterial graft was occluded in one other patient in each group (3% of arterial grafts). There was more postoperative blood loss, on average, in patients treated with aspirin, but the difference was not significant. Only one patient was withdrawn from long-term therapy because of possible gastrointestinal symptoms; most withdrawals from the trial were necessitated by commencement of aspirin or non-steroidal anti-inflammatory therapy for musculo-skeletal disorders. CONCLUSIONS The coronary bypass graft occlusion rate six months after surgery was low, and was lower on average in aspirin treated subjects but not significantly so. Long-term treatment with low-dose aspirin is recommended unless contraindicated.