We assessed the feasibility of low energy endocardial defibrillation in patients with atrial fibrillation or atrial flutter who had failed a trial of pharmacological reversion with amiodarone. Low energy endocardial defibrillation under general anesthesia was attempted in 9 patients, 5 with atrial flutter and 4 with atrial fibrillation (median duration of arrhythmia 3.75 months). Two large surface area endocardial leads were introduced percutaneously and sited in the right atrial appendage and at the right ventricular apex. A cutaneous patch electrode was placed on the left thorax. Biphasic shocks synchronized to the ventricular electrogram were used to terminate atrial arrhythmias. Three electrode configurations were evaluated in the following sequence at each energy level: atrial cathode to ventricular anode; ventricular cathode to atrial anode; atrial cathode to a combined ventricular and cutaneous anode. If endocardial defibrillation failed (0.5-10 J), transthoracic defibrillation using 200 joules followed by 360 joules, if required, was performed. Endocardial defibrillation was successful in all five patients with atrial flutter (0.5 J, 1.0 J, 1.0 J, 4.0 J, and 10.0 J) but in only one patient with atrial fibrillation (10 J). On no occasion did successful defibrillation occur with one configuration when it had failed with an alternate configuration at that particular energy level. Ventricular fibrillation did not occur, and there were no other significant complications. Low energy endocardial defibrillation is feasible in patients with atrial flutter using large surface area electrodes. Although the success rate of atrial defibrillation was low, further work is required, particularly in patients with more recent onset of the arrhythmia and using a right to left electrode configuration.