OBJECTIVE This study was designed to evaluate the efficacy of intracardiac cardioversion in patients with chronic atrial fibrillation after unsuccessful external cardioversion. BACKGROUND Previous studies in patients with atrial fibrillation undergoing intracardiac cardioversion have suggested that intracardiac cardioversion is highly effective and safe. However, the characteristics of patients who benefit most from this invasive technique are unknown. METHODS We prospectively studied 25 consecutive patients with chronic atrial fibrillation (11 +/- 9 months). All patients had undergone at least three attempts at conventional external transthoracic cardioversion by means of paddles in an anteroposterolateral position applying energies up to 360 J without success. Intracardiac shocks were delivered by an external defibrillator through defibrillation electrodes placed in the right atrium and coronary sinus or in the right atrium and left pulmonary artery. After conversion, all patients were treated orally with sotalol (mean 194 +/- 63 mg/day). RESULTS Internal cardioversion was successful in 22 of 25 patients at a mean defibrillation threshold of 6.5 +/- 3.0 J. Mean lead impedance was 56.4 +/- 7.4 omega. No severe complications were observed. At a mean follow-up of 15 +/- 12 months, 12 (55%) of the patients treated successfully remained in sinus rhythm. CONCLUSIONS In patients with failed external cardioversion, internal cardioversion offers a new option for restoring sinus rhythm. Intracardiac cardioversion is an effective and safe method and can be easily performed in patients with minimal sedation.