Intraoperative mapping with a hand-held, roving electrode requires a sustained rhythm lasting 5 to 10 minutes. To overcome this limitation, a computerized mapping system that records from 60 epicardial electrodes simultaneously was used to study 16 patients with Wolff-Parkinson-White syndrome. A sock containing 6 rows of electrodes arranged concentrically from base to apex was placed over the ventricles. The total time from placing the sock to analyzing the most basal row of electrode recordings was 5 minutes. A 39 × 44-mm plaque containing 56 electrodes was then placed across the atrioventricular (AV) groove for detailed simultaneous mapping of the ventricle and atrium in the preexcited region identified from the most basal row of sock electrodes. During plaque placement and recording, the remaining sock recordings were analyzed and a complete isochronal epicardial map was drawn. The plaque recordings were then analyzed. This technique rapidly detects early activation at the AV groove as do other computer systems using only a band of electrodes around the AV groove. Also, complete epicardial mapping supplied important additional information. One patient with a posterior paraseptal accessory pathway had ventricular epicardial breakthrough below the strip recorded by the AV band. When more than 1 early activation site was present along the AV groove, complete maps allowed multiple pathways to be differentiated from normal activation fronts ascending from the bundle branches. Complete epicardial maps allowed the study of rapidly changing or short-lived electrical events including isolated premature impulses, initiation and termination of reciprocating tachycardia by pacing, entrainment and changing degrees of fusion created by pacing during reciprocating tachycardia, and ventricular responses during atrial fibrillation. Thus, computer-assisted mapping of the entire ventricular epicardium offers specific advantages over both hand-held probe mapping and computer mapping using only an AV band of electrodes. © 1986.