Two hundred twenty-three patients were randomly selected to receive propranolol, 10 mg orally every 6 hours, or to serve as controls after coronary artery bypass grafting. The study began at the time of discharge from the intensive care unit. Patients were ineligible if they had cardiac arrhythmias while in the intensive care unit, low cardiac output requiring catecholamine support, or bradycardia requiring a pacemaker. In the control group, cardiac arrhythmias for which treatment was necessary developed in 31 of 136 patients (23%), atrial fibrillation or flutter in 24 patients (18%), and ventricular arrhythmias in 7 (5%). In the group receiving propranolol, cardiac arrhythmias requiring treatment developed in 9 of 87 patients (10%), atrial fibrillation or flutter in 7 (8%), and ventricular arrhythmias in 2 (2%). The difference in frequency with which cardiac arrhythmias occurred between the two groups is significantly different (p less than 0.05). We conclude that propranolol is effective in the prevention of cardiac arrhythmias following coronary artery bypass grafting.