The activation sequence of the human atrium has been inferred previously from a limited number of atrial electrograms recorded sequentially with a single-point mapping system. In 10 patients with Wolff-Parkinson-White (WPW) syndrome, three form-fitted, flexible templates containing a total of 156 bipolar electrodes were fixed to the epicardial surface of both atria. Data were recorded continuously from all 156 electrodes simultaneously during normal sinus rhythm, left atrial pacing, right atrial pacing, and reciprocating tachycardia. In all 10 patients, the site of the accessory pathway correlated with the results of the preoperative electrophysiological study and of the standard intraoperative band electrode mapping. The accessory pathway was located in the left free wall position in 8 patients, the right free wall position in 1 patient, and the posterior septal position in 1 patient. In 4 of the 8 patients with left free wall pathways, activation maps of retrograde atrial activation during reciprocating tachycardia demonstrated a broad base of initial atrial depolarization. This finding suggests that some accessory pathways may have a broad band of insertion on the atrium, and supports our practice of wide dissection of the entire anatomical space associated with each pathway to avoid recurrences of WPW syndrome. Simultaneous global atrial-activation mapping in patients with WPW syndrome provides a clearer understanding of atrial activation during reciprocating tachycardia.