This prospective study evaluated the accuracy of phase analysis of scintigraphic imaging in defining the site of earliest ventricular activation in pediatric patients with electrophysiologic disorders. Twenty patients (10.8 +/- 5.5 years) with preexcitation (n = 16) or ventricular tachycardia (VT) (n = 4) were independently evaluated by phase image analysis and endocardial catheter mapping. The earliest phase angle (contraction), which was common to three scintigraphic imaging planes during preexcited sinus rhythm or VT, was compared with the earliest retrograde atrial activation during reciprocating tachycardia or the origin of VT, as defined by catheter mapping. Phase analysis of earliest contraction was concordant with catheter mapping of electrical activation in all 13 free-wall accessory connections and in three of four patients with VT. Inconclusive definition of activation occurred only in paraseptal accessory connections or VT. In conclusion, phase analysis accurately defines anomalous ventricular activation that is due to free-wall accessory connections or VT. In patients with complex anatomy or small size, phase analysis allows noninvasive localization of the anatomic substrates of tachycardia.