The understanding of the onset mechanisms of paroxysmal atrial fibrillation (AF) may help to develop preventive therapy. Specific heart rate (HR) patterns and autonomic changes immediately before the onset of paroxysmal AF are not fully investigated. We undertook the present study to assess HR and heart rate variability (HRV) changes before the onset of AF using 24-h Holter electrocardiographic analysis in patients without antiarrhythmic medication.In 27 patients, 48 episodes of AF, lasting more than 30s and preceded by sinus rhythm for more than 1h were analysed. The hour preceding AF was divided in 5- and 30 min blocks. HR was also analysed in the last 15 beats. In 21% of the episodes, HR decreased >or=5% in the last 5 min (defined as deceleration); it increased >or=5% in 37% (defined as acceleration). HR, standard deviation (SD) and SD corrected for RR interval changed significantly in the last 5 min in the total group. Acceleration and deceleration were already visible over 30-min blocks in both these subgroups; changes in SD were only seen in the accelerators. The number of atrial premature beats (PACs) increased before AF, most clearly in the accelerators. Spectral HRV analysis revealed no additional information.Changes in HR, SD, and an increased number of PACs herald AF from at least 30 min before onset, more pronounced in accelerators. Spectral HRV parameters are not useful to foresee AF onset. This has possible implications for device therapy.