The reproducibility of electrocardiographic (ECG) recordings in syncopal recurrences and the diagnostic role of nonsyncopal arrhythmias are not well known. The objective of this study was to analyse the reproducibility of the ECG findings recorded with implantable loop recorders in 41 patients with suspected neurally-mediated syncope who were included in the International Study on Syncope of Uncertain Origin-2 study and that had > or =2 events recorded by implantable loop recorders. In these patients, the electrocardiogram obtained with the first documented syncope (index syncope) was compared with other recorded events. Twenty-two patients had > or =2 syncopes, and their electrocardiograms were reproducible in 21 (95%): 15 with sinus rhythm, 5 with asystole, and 1 with ventricular tachycardia; 1 had asystole at first syncope and sinus rhythm at recurrent syncope. In 32 patients with nonsyncopal episodes, an arrhythmia was documented in 9, and all of them had the same arrhythmia during the index syncope (100% reproducibility); conversely, when sinus rhythm was documented (23 patients) during nonsyncopal episodes, an arrhythmia was still documented in 6 during the index syncope (70% reproducibility; p = 0.0004). In conclusion, the ECG findings during the first syncope are highly reproducible in subsequent syncopes. The presence of an arrhythmia during nonsyncopal episodes is also highly predictive of the mechanism of syncope, but the presence of sinus rhythm does not rule out the possibility of arrhythmia during syncope. Therefore the finding of an arrhythmia during a nonsyncopal episode allows the etiologic diagnosis of syncope, and eventually to anticipate treatment, without waiting for syncope