In 20 patients with recurrent episodes of lone paroxysmal atrial fibrillation we assessed the onset pattern of each episode of either atrial fibrillation or of atrial flutter during a 24-h Holter monitoring. We evaluated 24 twenty-four-hour Holter tape recordings and our data are related to 168 episodes of paroxysmal atrial fibrillation and 27 episodes of paroxysmal atrial flutter. Eighty-five percent of atrial fibrillations and 67% of atrial flutters were of short duration (less than 5 min). The majority of patients (80%) had either nocturnal or daily episodes of arrhythmia and PP intervals immediately before onset of arrhythmia did not show significant variations in 77% of cases. The coupling interval of the supraventricular premature beats eliciting atrial fibrillation was significantly shorter than the coupling intervals of the spontaneous isolated supraventricular premature beats (p less than 0.0001); again, in 6 patients with either atrial fibrillation or flutter, the coupling interval at onset of fibrillation was significantly shorter in comparison to flutter (p less than 0.0001). In conclusion, vagal or sympathetic prevalence does not seem to influence significantly the beginning of the arrhythmia, while the coupling interval of the atrial premature beats plays a critical role in the inducibility of atrial flutter or fibrillation.