Patients scheduled to receive at least three aortocoronary venous bypass grafts were randomized to active medication or to placebo. The former were given dipyridamole (DP) preoperatively and acetylsalicylic acid (ASA) was added after the operation. For the next 3 months they received DP 75 mg and ASA 325 mg thrice daily. The placebo regimen was identical and the study was conducted with double-blind technique. One patient in each group died. DP-ASA was discontinued in six patients because of gastrointestinal side effects (bleeding peptic ulcer in 2 cases). Angiography after 3 months revealed the patency rate of individual grafts to be 68% in the DP-ASA group and 77% in the placebo group. DP-ASA therefore did not prevent occlusion of aortocoronary venous bypass grafts.