Cardiac arrhythmias are a frequent complication after thoracic operations. The prophylactic value of flecainide administered as a constant-rate, intravenous infusion (0.15 mg/kg/h) after a loading dose (2 mg/kg) was tested in a randomized, placebo-controlled study in 30 patients using Holter monitoring during the first 72 hours after operation. A high incidence of atrial and complex ventricular arrhythmias was observed in the placebo-treated group, with a clear predominance in patients undergoing left thoracotomy. Antiarrhythmic drugs had to be added in 6 patients in the placebo-treated group (n = 16), but in none in the flecainide-treated group (n = 14) (p less than 0.01). The preventive effect of flecainide was observed at serum concentrations in the middle of the therapeutic range (409 ng/mL) without side effects. It is concluded that flecainide administration effectively reduced or prevented cardiac arrhythmias after thoracic operations.