Introduction: Stepwise radiofrequency catheter ablation (step-CA) has become a treatment of choice for the restoration of sinus rhythm (SR) in patients (pts) with long-standing persistent atrial fibrillation (pers-AF). Its success rate appears limited as the amount of ablation to achieve long term SR is unknown. Recently, intracardiac organization indices (OI) of AF have been used to track the efficiency of step-CA, with limited success. Our study is aimed at developing new OIs based on the relationships between harmonic components of atrial activity from the surface ECG as a global assessment of AF complexity and organization during step-CA. Methods: 3 pts with pers-AF (age 62, AF duration 17 months) underwent a step-CA. An adaptive tracking algorithm was developed for estimating the instantaneous frequency of atrial activity on chest lead V1 (after QRST subtraction) and for extracting its fundamental and harmonic components. An adaptive organization index (AOI) was computed as the ratio between the power of the extracted components and the total power of the signal to evaluate the temporal evolution of AF oscillations. The variance of the phase difference (PD) between the fundamental and harmonic components was used for measuring AF regularity. Results: Step-CA terminated 2/3 pers-AF into flutter. Importantly, in the 2 terminated pts, the AOI did not show any significant change during the step-CA (from pre-ablation to CFAE), while the PD showed a gradual reduction suggestive of increased coupling between the fundamental and the 1st harmonic. See figure. Conclusions: The PD and the AOI as measurements of complexity from the surface ECG appear as promising methods for tracking the effect of step-CA on global AF organization. This, however, needs to be validated on a larger population.