Fragmented atrial activity in patients with transient atrial fibrillation. Prediction of atrial fibrillation (AF) is very important in patients with Wolff-Parkinson-White syndrome or in the selection of pacemaker therapeutic modality. In 25 patients with transient AF, the response of the atrial activity width to extrastimuli was examined in comparison with 25 patients without AF to see if the results could be used as an index of subsequent occurrence of AF. Programmed electrical stimulation using eight basic stimuli followed by single or double extrastimuli (P1P2 or P1P2P3) were delivered to the high right atrium, and the atrial activities were examined. The prolongation of the atrial activity caused by extrastimuli was termed fragmentation (Frg), and it was defined as the prolongation of more than 150% of the basic stimuli. Frg zone was defined as the zone of coupling intervals of the extrastimuli (P1P2 or P2P3) that caused Frg, and delta max Frg was defined as the difference between the widest Frg and the atrial wave width during basic stimuli. Fragmentation was reproducibly induced by extrastimuli, and there was an inverse relationship between Frg duration and the coupling interval of the extrastimuli (P1P2 or P2P3). Frg zone and delta max Frg were wider and longer in patients with transient AF in comparison with the control group for both single and double extrastimuli (p less than 0.001). AF inducibility using double extrastimuli was significantly high in patients with AF. 