Cardiac arrhythmias are known complications of thoracic operations. The prophylactic value of flecainide administered as a constant-rate, intravenous infusion (0.15 mg.kg-1.h-1) after a loading dose (2 mg.kg-1) was compared with digoxin (10 micrograms.kg-1 for 12 hours, then 0.25 mg.24 h-1) in a randomized study in 30 patients using Holter monitoring during the first 72 hours after operation. Drug monitoring was performed every day to keep a serum level of flecainide of 200 to 600 ng.mL-1 and a digoxin level of 0.8 to 2 ng.mL-1. Failure, defined as the appearance of atrial fibrillation or flutter or the development of complex ventricular arrhythmias (Lown IVb and V), was observed in one patient in the flecainide group (7%) and in 7 patients in the digoxin group (47%) (p less than 0.05). It is concluded that flecainide is more efficient than digoxin in preventing and treating cardiac arrhythmias after thoracic operations. At the dosage used side effects related to flecainide or digoxin were not observed.