To evaluate the role of antithrombotic therapy, on preserving graftpatency, we performed a meta-analysis of randomized clinical trialsinvolving aspirin (ASA), dipyridamole (D), anticoagulants (AC) and placeboor nontreatment controls (P). Manual literature searches were performedsupplemented by computerized MEDLINE listings complete to July 1991.Saphenous vein graft occlusion was determined by angiography (patients with> or = 1 distal anastomotic occlusion). The trial data were aggregatedwith the methods of Mantel and Haenszel. The results are reported as oddsratios (OR) +/- 95% confidence intervals (CI). Seventeen trials wereevaluated. Aspirin strongly influenced graft occlusion [ASA +/- D vs P: OR0.60, 95% CI 0.51, 0.71, P < 0.0001], but dipyridamole provided noadditional benefit [ASA+D vs ASA: OR 0.94, 95% CI 0.72, 1.24, P = 0.71].Anticoagulants reduced graft occlusion [AC vs P: OR 0.56, 95% CI 0.33,0.93, P = 0.025] and the results were similar to that achieved with aspirin[ASA vs AC: OR 0.95, 95% CI 0.62, 1.44, P = 0.87]. The combination ofaspirin and anticoagulants was superior to anticoagulants alone in twolimited trials [ASA+AC vs AC: OR 0.55, 95% CI 0.33, 0.88, P = 0.01]. A low(100 mg) to medium (325 mg) daily aspirin dosage was more effective than ahigh dose (975 mg). Early postoperative treatment (< or = 6 h) stronglyinfluenced graft occlusion while preoperative administration provided noadditional benefit. No mortality advantage was identified for anyantithrombotic therapy. Aspirin or anticoagulants enhance saphenous veingraft patency following aortocoronary bypass surgery, and a combinationthereof deserves further investigation in a trial large enough to detectthe effects of these treatments with respect to clinical events.