This study examined the clinical, echocardiographic, and electrophysiological factors influencing the atrial defibrillation threshold (ADFT) in patients with chronic, persistent AF undergoing transvenous, low energy, atrial cardioversion. Twenty-two patients (age 57 +/- 15 years) with a mean AF duration of 7.8 +/- 7.1 months (range 2-32 months) underwent internal cardioversion with catheters placed in the right atrium and coronary sinus. Biphasic shocks (3/3 ms) were delivered in a step-up protocol. ADFT was defined as the lowest energy shock that converted AF to sinus rhythm. All patients were successfully cardioverted at a mean ADFT of 5.62 +/- 2.82 J (range 2.6-12.9 J). Fifteen variables, including clinical characteristics (age, body mass index, AF duration, etiology), echocardiographic measurements (atrial diameter and volumes, indexes of ventricular performance), hemodynamic measurements, and mean atrial cycle during AF were analyzed as possible predictors of ADFT. In univariate regression analysis, AF duration, mean RR interval, and cardiac index correlated with ADFT. In multivariate regression analysis, AF duration remained as the only significant predictor of ADFT (B coefficient 0.311, P < 0.001; 95% confidence interval [CI] 0.194-0.427). AF duration was the most powerful predictor of ADFT. It should be considered when planning internal CV of AF to limit the number of shocks delivered. Furthermore, long intervals between AF onset and CV should be avoided.