Phase image analysis (first Fourier harmonic transformation) has been performed in 5 men with WPW syndrome to define the abnormal patterns of ventricular emptying during sinus rhythm and transesophageal pacing at different rates. All patients but one showed basal ventricular preexcitation. Of the 4 patients with basal ventricular preexcitation the earliest ventricular emptying occurred in the left ventricular free-wall in 1 patient and in the right ventricular free-wall in 3 patients. In the patient without ventricular preexcitation at rest transesophageal pacing at a rate of 100 bpm induced first ventricular activation in the left lateral ventricular free-wall while at a rate of 120 bpm it returned to normal. In the patients with ventricular preexcitation at rest, the basal image abnormalities become more evident as preexcitation was augmented. Of interest, in 1 patient with basal type B ventricular preexcitation the sequential phase image analysis, at a rate of 100 bpm confirmed the earliest ventricular activation in the right ventricular free-wall while at a rate of 120 bpm showed the earliest emptying in left ventricular free-wall suggesting the presence of 2 accessory connections. We conclude that phase mapping combined with transesophageal pacing may be a useful and reliable method to localize single as well as multiple accessory pathways in patients with ventricular preexcitation.