BACKGROUND: Paroxysmal atrial fibrillation (PAF) causes not only severe symptoms and hemodynamic changes, but may progress to chronic atrial fibrillation. Autonomic nervous system or atrial premature beat (APB) has been suggested to contribute to the spontaneous initiation of PAF, but the exact mechanism has been largely unknown. METHODS: One hundred and twenty nine episodes of PAF lasting longer than 5 sec were analyzed in 18 patients (M:F=11:?). Two minutes of normal sinus rhythm before the onset of PAF, and the initial one minute of PAF were printed and analyzed. RESULTS: Most of PAFs were initiated by APBs (38%) or rapid atrial tachycardias (AT, 59%). The frequency of APBs tended to increase immediately before PAF onset (p=0.08). The coupling intervals and coupling indices were not significantly different between PAF-producing APBs and benign APBs. More than half of PAF episodes were initiated by rapid ATs (rate, 357+/-50 bpm). After the onset, they accelerated over several seconds and then degenerated into AF. In some cases, transition from AF to atrial flutter and vice versa were observed. Heart rate, measured at 60-second intervals during 2 minutes before PAF onset, did not change significantly (p=0.44). CONCLUSION: Most of PAFs were initiated by APBs or rapid ATs. Heart rate did not change significantly but the frequency of APBs tended to increase immediately before PAF onset. Rapid ATs frequently accelerated and degenerated into AF. In this regard, Holter monitoring could be useful in identifying patients with PAF triggered by rapid ATs.