Eighty-three patients with paroxysmal supraventricular tachycardia (Wolff-Parkinson-White syndrome in 47 patients and atrioventricular nodal reentry in 36 patients) underwent electrophysiologic study before and after intravenous administration of flecainide. Sixty-seven of these 83 patients entered an open clinical trial to evaluate the efficacy and safety of long-term oral treatment with flecainide. Efficacy was evaluated by a patient diary of spontaneously occurring attacks during flecainide treatment and periods of withdrawal. In 24 patients, electrophysiologic study was repeated during oral therapy. During a mean follow-up of 22.8 months, the therapeutic response to flecainide (mean dose 263 mg/day) was good in 48 patients (71%) and fair in 7 patients (10%). In 8 patients (10%), an arrhythmogenic effect was seen (increased number and prolonged duration of supraventricular tachycardia episodes in 4 patients, spontaneous episodes of atrial fibrillation in 2 patients and sinoatrial block in 2 patients). No consistent abnormalities were seen in the laboratory tests. Extracardiac side effects were of minor importance and usually subsided spontaneously or disappeared after reduction of dosage. It is concluded that flecainide is a useful addition to the drugs already available for short- and long-term treatment of supraventricular tachycardia due to Wolff-Parkinson-White syndrome and atrioventricular nodal reentry.