Background-The mechanisms of chronic atrial fibrillation (AF) are not well understood. We performed epicardial mapping of chronic AF in patients undergoing open heart surgery to test the hypothesis that chronic AF is due to a left atrial "driver" with a regular, short cycle length, resulting in fibrillatory conduction to the rest of the atria. Methods and Results-Nine patients with chronic AF (1 month to Ͼ15 years' duration) were studied at open heart surgery, 8 before and 1 during cardiopulmonary bypass. During AF, atrial electrograms (AEGs) were recorded for 1 to 5 minutes from 404 epicardial electrodes arranged in bipoles along with ECG lead II or ventricular electrogram. Four-second segments of each bipolar AEG were also subjected to fast Fourier transform analysis. Two patterns of atrial activation were present during AF. In pattern 1 (7/9 patients), AEGs from parts of the atria demonstrated a short, regular cycle length with identical beat-to-beat morphology, and the rest of the atria were activated irregularly, and AEGs that demonstrated constant morphology and cycle length were localized to parts of the left atria (5/7), the right atria (1/7), or both atria (1/7). In pattern 2 (2/9 patients), AEGs showed no evidence of regular activation or constant morphology. Conclusions-In 9 patients with chronic AF, the commonest recorded AEG pattern showed an area of regular, rapid rhythm, consistent with the possibility that a driver causing fibrillatory conduction is one mechanism of AF in these patients.