Background Supraventricular tachycardia (SVT) is a common tachyarrhythmia among pediatric patients. Usually, non-preexcited SVT is attributable to either atrioventricular nodal reentry tachycardia (AVNRT) or unidirectional retrograde accessory pathway (URAP), but these cannot be differentiated on a baseline electrocardiogram (ECG). The ability to identify the SVT mechanism in children may guide decision-making about treatment and counseling regarding electrophysiology study (EPS). Clinical experience suggested that ectopic atrial rhythm (EAR) is more frequently observed on preablation ECGs of pediatric patients with AVNRT. This study aimed to determine whether EAR is predictive of AVNRT. Methods A 10-year single-center retrospective review was conducted with patients who underwent ablation for SVT from 1997 through 2006. All pediatric patients with documented AVNRT or URAP during EPS were included. The exclusion criteria specified prior ablation, Wolff-Parkinson-White syndrome, or complex congenital heart disease. A patient was considered to have EAR if a preablation ECG had a p-wave axis less than 0° or greater than 90° or a wandering atrial pacemaker with at least three different p-wave morphologies. Results The review found 457 eligible patients ages 0.5 to 21 years: 285 with AVNRT and 172 with URAP. Patients with congenital heart defects represented 5.6% of the AVNRT group and 2.9% of the URAP group. Ectopic atrial rhythm was seen in 45 (16%) of 285 patients with AVNRT compared with 10 (6%) of 172 URAP patients (p = 0.001). The sensitivity and specificity of EAR for AVNRT was 16% and 94%, respectively, and the positive predictive value was 82%. There was no difference in heart rate or QRS duration between the two groups. Conclusion On preablation ECG for pediatric SVT patients, EAR is a reasonably specific marker for AVNRT.