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Ectopic atrial activity may be a mechanism of onset of some forms of atrial fibrillation (AF). This ECG from a 77 year old man shows triggering and termination of AF. AF is present at the start but rapidly terminates. During sinus rhythm atrial extrasystoles (arrows) reinitiate short bursts of AF. A third atrial extrasystole (*) is not conducted. The P wave morphology and axis of the triggering ectopic beats suggested focal activity close to the right superior pulmonary vein. During a treadmill exercise test AF was initiated by an atrial extrasystole and sustained for 16 minutes. Subsequent Holter monitoring revealed frequent atrial ectopy (239 per hour) and 11 episodes of AF lasting from a few seconds to 4.8 minutes. No other aetiology for AF was found. Recognition of this focal mode of onset of AF in the otherwise normal heart is important since radiofrequency ablation is potentially curative.