BACKGROUND Circadian variation in the incidence of acute cardiovascular events is well known but has not been extensively investigated in paroxysmal atrial fibrillation, although the significance of this arrhythmia is growing in our society with the increasing number of aged people. RESULTS We detected 150 patients with paroxysmal atrial fibrillation in a drug-free state from among 25,500 consecutive Holter recordings. To determine whether the onset, maintenance, and termination of paroxysmal atrial fibrillation were random events, we analyzed the total recorded duration of arrhythmia and the incidence of and number of patients with the onset, maintenance, and termination of this arrhythmia as hourly data and as hourly probabilities. A prominent circadian rhythm of the total duration of atrial fibrillation, approximately 90% of which was well explained by a single cosinusoidal function, was detected with a nadir around 11 AM. Because the onset of the arrhythmia had little or no circadian rhythm, this finding was due to a diurnal pattern of maintenance and termination, both of which were well expressed by a double-harmonic density function. Maintenance showed a trough at 11 AM, and termination showed a peak at the same time, leading to the nonuniform duration of single episodes of atrial fibrillation throughout the 24-hour day. CONCLUSIONS Paroxysmal atrial fibrillation showed a unique circadian variation that differed from the well-known pattern for acute cardiovascular events, a point that should be kept in mind when antiarrhythmic therapy is evaluated. Identification of factors that regulate the circadian pattern of the maintenance and termination of paroxysmal atrial fibrillation may lead to better chronotherapy for preventing perpetuation of this arrhythmia.