It was recently shown that atrial fibrillation (AF) waves on chest lead V1 adequately reflect right atrial appendage (RAA) activity during long standing persistent AF (pers-AF). The contribution of the left atrial (LA) activity to chest leads AF waves, however, remains unknown. Our study is aimed at evaluating the respective contribution of the RA and LA depolarization to ECG chest leads AF waves during pers-AF. Methods: Catheters (CAT) were introduced in 10 consecutive patients (60±5 y, AF duration 22±14 m) prior to ablation: 1) a quadripolar CAT in the RAA, 2) a decapolar CAT in the coronary sinus (CS) and 3) a duodecapolar CAT in the LA appendage (LAA). Local activation times were extracted from bipolar recordings using sliding windows. Chest lead V6 was placed in the back (V6b). Mean AF cycle length (AFCL) of leads V1 to V6b were computed as the inverse of the dominant frequency of ECG spectra after QRST cancellation, and compared to intracardiac RAA, LAA and CS AFCL using Pearson’s correlation coefficient. Results: The figure shows that the correlation between RAA and chest leads AFCL was maximal for V1 and progressively dropped till V5, with a moderate rise for V6b. LAA AFCL showed the opposite pattern with the highest correlation in V6B and the lowest one in V2. The correlation of CS AFCL was similar to the LAA one, but of lower magnitude. Conclusion: Our preliminary results suggest that the respective contribution of RAA and LAA activities can be estimated using a modified surface ECG. Whether this technique has the potential to guide ablation of LA and RA drivers in pers-AF needs further validation.