Precise localization of accessory pathways (APs) is crucial to minimize radiofrequency (RF) energy applications in the Wolff-Parkinson-White (WPW) syndrome. Although several markers have been described for identifying APs, no gold standard has thus far been established. The present study attempted to validate the hypothesis that an early intrinsic deflection (ID) would be identifiable in the unipolar ventriculogram, if this was recorded at or near the site of endocardial breakthrough of the AP. The electrograms of 23 patients with the WPW syndrome who underwent RF ablation were analysed using a computer-based system. A total of 50 electrograms (19 successful and 31 unsuccessful RF energy applications) were studied. The downstroke of the unipolar ventriculogram was measured at 1 msec intervals for the dV/dt; the maximal dV/dt (the most rapid segement of the downstroke) was considered as the ID. The following parameters were found to differentiate between successful and unsuccessful RF ablation attempts: (i) Timing of the ID relative to the delta wave onset (ID-delta = plus 11 +/- 21 msec versus minus 18 +/- 22 msec, p < 0.001). (ii) Timing of the ID relative to the onset of the unipolar ventriculogram (Vu-ID = 14 +/- 7 msec versus 29 +/- 15 msec, p < 0.001). (iii) Maximal dV/dt in the initial 20 msec of the unipolar ventriculogram (367 +/- 146 microV/msec versus 207 +/- 97 microV/msec, p < 0.001). The other parameters (probable AP potential, bipolar ventriculogram timing, continuous electrical activity, unipolar signal morphology) were not helpful in this regard. Hence, the identification of the ID and measurement of its timing is helpful in localising overt APs for successful delivery of RF energy.