Internal Cardioversion.introduction: This report describes the electrical conversion of atrial fibrillation in two morbidly obese patients refractory to external cardioversion at 360 J.Methods and Results: The two patients were lightly sedated and underwent placement of decapolar catheters in the coronary sinus and right atrial appendage. All ten electrodes of each decapolar catheter were electrically coupled, and defibrillation was attempted at successively increasing levels using a biphasic decaying exponential waveform generated by an external defibrillator. Both patients were returned to normal sinus rhythm using<10 J without complication.Conclusion: Internal cardioversion is effective in restoration of sinus rhythm in some patients refractory to conventional forms of therapy