The effect of sulfinpyrazone on the incidence of early postoperative closure of saphenous vein bypass grafts was compared with placebo in a prospective randomized study of 255 eligible patients. Treatment with sulfinpyrazone (800 mg/day) was started 24 hours after operation in 130 patients; 125 patients received placebo. Graft blood flow was measured at operation in 96 percent of all patients. Graft angiography was performed between the 7th and 14th postoperative days. There was no significant difference between the two groups in graft blood flow, number and diameter of the grafted arteries, left ventricular filling pressure or ejection fraction. During the study 73 patients (41 on sulfinpyrazone, 32 on placebo therapy) were excluded because graft angiography was contraindicated or because of concomitant use of anticoagulant or antiplatelet drugs. The incidence rate of early graft closure in the remaining 182 patients (43.1 grafts) was 3.8 percent (8 or 212) in the sulfinpyrazone group and 9.1 percent (20 of 219) in the placebo group (p less than 0.025). The incidence of graft closure for the sulfinpyrazone and placebo groups classified according to the recipient coronary arteries was: (1) left anterior descending artery; 3 of 98 versus 11 of 111; p less than 0.05; (2) left circumflex coronary artery: 3 of 50 versus 5 of 43; difference not significant; (3) right coronary artery: 2 of 64 versus 4 of 65; difference not significant. The incidence of closure in grafts with a flow of less than 30 ml/min did not differ significantly in the sulfinpyrazone and placebo groups (4 of 26 versus 6 of 22). These results suggest that sulfinpyrazone reduces the incidence of early graft closure in grafts with a flow rate greater than 30 ml/min.