The clinical efficacy, adverse effects and pharmacokinetics of flecainide were evaluated in 16 pediatric and young adult patients with supraventricular tachycardia (SVT). Patients had received an average of 2.8 drugs before flecainide was tried. The following mechanisms of supraventricular arrhythmias were determined in patients by intracardiac electrophysiologic studies: atrioventricular node reentry, 4; reentry through an accessory connection, 7; atrial automatic focus, 2; atrial flutter, 3. Twelve patients had normal cardiac anatomy and 4 had congenital heart disease. Each patient received 2.8 mg/kg/day of flecainide divided into 2 doses 12 hours apart. After 3 days, the dose was increased to 5.6 mg/kg/day if necessary. In 14 patients, serum flecainide concentrations measured 3 to 4 days after beginning therapy ranged from 0.1 to 0.8 micrograms/ml (mean 0.40). Flecainide successfully controlled SVT in 8 of 16 patients. SVT in 3 of 7 patients with accessory connections and in 3 of 4 patients with atrioventricular node reentry was successfully controlled. In 1 of 2 patients with atrial automatic tachycardia, SVT had been completely controlled over 16 months. Only 1 of 3 patients treated for atrial flutter responded. Follow-up for successfully treated patients ranged from 4 to 16 months (median 9). Seven patients continue to take flecainide. None of the patients had clinical congestive heart failure. No drug-related adverse effects were noted on the resting surface electrocardiogram. Flecainide rarely produced proarrhythmic effects in this series. The 2 that were observed were mild and caused no clinical problems. Noncardiovascular side effects also occurred infrequently.(ABSTRACT TRUNCATED AT 250 WORDS)