In most patients, atrial fibrillation (AF) is initiated by pulmonary veins foci. We investigated whether these left atrial events are reflected in the surface electrocardiogram (ECG) and whether additional information can be obtained from recording posterior leads in patients with AF. In 7 patients, we identified 51 5-second segments with a significant frequency gradient between right (RA) and left (LA) intra-atrial electrograms, as well as frequency changes from segment to segment in the same patient. QRS-T cancellation methods were used to isolate atrial activity in the surface ECG, and peak frequencies were computed. Peak frequencies of posterior leads were very similar (6.0 ± 1.3 Hz for V7, 5.9 ± 1.4 Hz for V8, 6.0 ± 1.1 Hz for V9, 6.0 ± 1.3 Hz for V10). Lead V9 seemed to have the best signal quality and was chosen for further analysis. We found a strong correlation between V1 and RA and between V9 and LA, 0.98 and 0.93 respectively, while the lowest correlation was found between lead V1 and LA, 0.64, p < 0.00001. Magnitude-Squared Coherence values were highest between V1 and RA and lowest between V1 and LA.