Background - Atrial fibrillation (AF) is uncommon in children, and its mechanisms are unknown. This study describes the electrophysiological findings in children and adolescents with AF and the outcome of catheter ablation. Methods and Results - Nine adolescents with symptomatic, one AF who failed antiarrhythmic drug therapy were evaluated. All patients had ECG-documented AF and underwent invasive electrophysiological testing. Intracardiac mapping was performed to determine the site of spontaneous onset of AF and rapidly firing atrial foci. Only the triggering focus was targeted for ablation or isolation. The patients' mean age was 15.9±3.3 (range, 8 to 19 years). The most common finding was rapid, irregular atrial tachycardias in the region of the pulmonary veins (n=5), left atrium (n=2), or crista terminalis (n=3). One patient had foci in both the pulmonary veins and crista terminalis. The cycle lengths ranged from 108 to 280 ms. Catheter ablation was acutely successful in 8 patients (88.9%), whereas 1 patient with multiple left atrium foci was treated with the surgical maze operation. Over a mean of 35±22 months, 7 patients (77.8%) were arrhythmia free on no medications, while AF recurred in 2 patients who are controlled on antiarrhythmic medications. Two patients with tachycardia-induced cardiomyopathy had resolution of their left ventricular dysfunction after ablation. Conclusions - AF in adolescents with structurally normal hearts is usually due to foci in the pulmonary veins, crista terminalis, or left atrium. These foci usually induce irregular atrial tachycardias. Catheter ablation of the foci is effective in eliminating recurrent AF.