Twenty patients with the Wolff-Parkinson-White (WPW) syndrome and 1 or more episodes of symptomatic atrial fibrillation (AF) due to rapid anterograde bypass tract conduction underwent electrophysiologic testing. The mean ventricular rate during spontaneous AF was 242 +/- 56 beats/min (+/- standard deviation) and the shortest preexcited R-R interval was 194 +/- 40 ms. Six patients underwent surgical bypass tract ablation and 14 were treated medically, based on the results of electropharmacologic testing. Over a mean follow-up period of 35 +/- 19 months (+/- standard deviation), only 1 patient treated medically had a recurrence of minimally symptomatic AF. The successful chemoprophylaxis of symptomatic AF was associated with the inability to induce AF and atrioventricular reciprocating tachycardia during drug testing (7 patients) or with the induction of AF with a ventricular rate less than 200 beats/min and a shortest preexcited R-R interval of greater than 250 ms (7 patients). Electrophysiologic testing can identify a subgroup of patients with WPW and AF in whom medical therapy is a suitable alternative to bypass tract ablation.