Background - This study tested the ability of sequential shocks delivered through dual-current pathways to lower the atrial defibrillation threshold (ADFT) compared with a biphasic shock through a standard single- current pathway. Methods and Results - Electrodes were positioned in the right atrial appendage (RA), left subclavian vein (LSV), proximal coronary sinus (CSos), and distal coronary sinus (DCS) in 14 patients with chronic atrial fibrillation (170±185 days). Using a step-up protocol, we compared ADFTs for a single-current pathway (RA→DCS) that used a single 7.5/2.5-ms biphasic shock from a 150-μF capacitor with those for a dual-current pathway system (RA→DCS followed by CSos→LSV) using sequential 7.5/2.5-ms biphasic shocks with capacitor discharge waveforms for 150-μF and 600-μF capacitors. Both dual-current pathway configurations (2.0±0.4 J for 150-μF capacitance, 2.4±0.5 J for 600-μF capacitance) had a significantly lower ADFT than the single-current pathway (5.1±q-1.8 J). Whereas the dual-current pathway with 150-μF capacitor shocks had a significantly lower energy threshold, there was no statistical difference in terms of leading-edge voltage compared with the dual-current pathway with 600-μF capacitance shocks. There were no ventricular arrhythmias induced with appropriately synchronized shocks. Conclusions - For internal atrial defibrillation in humans, sequential biphasic waveforms delivered over dual-current pathways resulted in a markedly reduced (>50% reduction) ADFr compared with a single shock over a single-current pathway.