The outcome of stepwise catheter ablation (step-CA) for patients (pts) with long-standing atrial fibrillation (LS-pAF) is poorly predicted by classical organization indices (OI). Our study aims at developing new indices from pre-ablation ECG in order to identify the site of AF termination during step-CA. Methods: 23 consecutive pts (59±7y, AF duration 19±12m) underwent step-CA consisting in pulmonary veins isolation, left atrial (LA) defragmentation, and right atrial (RA) ablations for non terminated AF. Chest lead V6 was placed on pts’ back within the cardiac silhouette (V6b). QRST cancellation was performed on chest leads V1 and V6b. Using an adaptive harmonic frequency tracking scheme, two indices were computed to quantify the harmonic components of atrial activity: 1) PD, phase difference between the AF dominant frequency (DF) and its 1st harmonic measuring AF regularity, and 2) AOI, an adaptive OI tracking AF temporal evolution. Both indices were compared to two classical OIs: 1) SOI, ratio of the DF power and its 1st harmonic to the total power (3-15 Hz) and 2) AFCL, ECG AF cycle length. Results: LS-pAF was terminated in 83% (19/23) of the pts: 17 during LA (LT); 2 during RA ablation (RT); 4 were not terminated (NT). The figure shows that AOI on lead V1 (panel A) and PD on lead V6b (panel B) improved the separation between LT and RT/NT pts compared to SOI and AFCL respectively. AFCL was not significantly different between the two groups. Conclusions: Our results suggest that adaptive measures of AF organization computed from pre-ablation ECG perform better than classical indices for identifying pts whose AF will terminate during ablation within the LA. These finding are indicative of a higher baseline organization in these pts that could be used to select LS-pAF candidates for step-CA.