Purpose: multiple organization indices (OIs) have been used to predict the outcome of stepwise catheter ablation (step-CA) in long-standing persistent AF (pers-AF), however with limited success. Our study aims at developing innovative OIs from baseline (BL, before ablation) ECG in order to predict the outcome of step-CA. Methods: fourteen consecutive patients (pts) (60±5 y, AF duration 21±9 m) underwent a step-CA consisting in pulmonary veins isolation, left atrial (LA) defragmentation and linear ablations, and right atrial (RA) ablations if non terminated. Chest lead V6 was placed in the back (V6b). After QRST cancellation from chest leads V1 to V6b, two OIs were computed to quantify the harmonic components of ECG atrial activity: 1) phase difference variance (PD) between the AF harmonic components as a measure of AF regularity, and 2) adaptive OI (AOI) evaluating the time evolution of the AF harmonic components. Both indices were compared to classical ones: a spectrum-based OI (SOI) and ECG AF cycle length (AFCL). Results: Pers-AF was terminated into sinus rhythm or atrial tachycardia in 10/14 pts during step-CA, 8 during LA (LT), 2 during RA (RT) ablation, and 4 were non terminated (NT). The figure shows that LT was best separated from RT/NT before ablation by AOI computed on lead V1 (A) and PD from lead V6b (B) as compared to SOI and AFCL respectively. Conclusion: our results suggest that adaptive OIs computed before ablation perform better than classical OIs for separating LT from RT/NT pts. These findings are indicative of a higher baseline organization in LT pts that could be used to select candidates for the restoration of sinus rhythm by step-CA.