Supraventricular tachycardia (SVT) is the most common cardiac arrhythmia in children. Atrioventricular (AV) reciprocating tachycardia utilizing an accessory pathway and AV nodal reentrant tachycardia (AVNRT) constitute a majority of SVTs. Radiofrequency energy ablation has recently become the treatment of choice in adults. Initial experience in pediatric patients in this Institute is reported in this study. Six consecutive patients (aged 5-22 years, mean age 13 years; 3 boys, 3 girls) with narrow-QRS SVT were studied from July to October in 1993. They had experienced symptomatic SVT for a mean period of five years (ranged from 1 to 15 years). Anti-arrhythmic agents, including beta-antagonists and calcium channel blockers, were unable to satisfactorily suppress these SVTs. Four of the six patients were shown to have a left-sided concealed accessory connection as the retrograde limb of the reentrant circuit. After identification of the earliest retrograde activation site, a steerable 7F catheter with a 4-mm-long electrode at the distal tip was placed within the left ventricle and positioned against the atrio-ventricular annulus. Another two patients were proved to have AVNRT. The ablation catheter was placed at the posterior/inferior aspect of the tricuspid annulus and guided by putative slow potential. Radiofrequency current was delivered at a power from 15 to 25W for four to six times with a mean duration of 40 seconds in patients with AV reciprocating tachycardia, and a single time in those with AVNRT. All six SVTs were successfully ablated. No complication was noted in any patient. Follow-up for three to seven months has indicated no recurrence of SVT.(ABSTRACT TRUNCATED AT 250 WORDS)