It is commonly assumed that the presence of high frequency components in body surface potentials implies that fractionated activation fronts, caused by heterogeneously viable tissue, are present in the heart. However, it is possible that non-fractionated activation fronts can also give rise to high frequency surface potentials and that the relative amount of high frequency power is related to the complexity of the activation sequence. In a test of this idea, averaged body surface potentials were recorded during the entire QRS complex of nine Wolff-Parkinson-White (WPW) patients in situations in which fractionated activation fronts should not have been present, but which represent increasing degrees of complexity of ventricular activation: (1) postoperative ectopic pacing from subepicardial wires placed during surgery, when a single coherent activation front was present throughout most of the QRS; (2) Preoperative preexcited rhythm, when a single coherent activation front was present for one portion of the QRS (the delta wave); and (3) postoperative normal rhythm, when two or more activation fronts were present in the ventricles throughout most of the QRS. For comparison, averaged body surface potentials were also analyzed during the last 40 ms of the QRS complex and the ST segment of 14 postinfarction patients with chronic ventricular tachycardia. In the patients with WPW syndrome, relatively high frequency content increased (attenuation -36.7 vs -27.2 vs -18.3 dB) and QRS width decreased (160.7 vs 125.9 vs 94.1 ms) significantly from paced to preoperative to postoperative beats. Significant high frequency content was present in all cases, showing that coherent activation fronts can give rise to high frequencies. Interestingly, the postoperative QRS of WPW patients contained a larger proportion of high frequency power than did the late potentials of the patients with ventricular tachycardia. Thus, while the presence of late fractionated body surface potentials may be a marker for ventricular tachycardia, these potentials by themselves do not necessarily signify that the underlying cardiac activation giving rise to these signals is fractionated.