To prospectively assess the incidence of cardiac dysrhythmias before and after closure of atrial septal defects (ASDs) using the Amplatzer septal occluder (ASO), 24-hour Holter electrocardiograms (ECGs) were performed before and 1 year after the procedure in 23 pediatric patients (9 male and 14 female). Patients' ages ranged from 2 to 15 years (mean, 7.1 years). All had an ASD of the secundum type that was completely closed (n = 22) or had a small residual shunt (n = 1). No preexisting dysrhythmia was present in 22 patients; atrioventricular nodal reentrant tachycardia had been diagnosed in 1 patient. During the observation period, no clinical dysrhythmia occurred. Analysis of the Holter ECGs before the intervention showed regular sinus rhythm in 20 patients and sinus rhythm with intermittent atrial rhythm in 3 patients. Atrial premature complexes (APCs) were detected in 1 patient, and a ventricular couplet was present in 1 patient. The Holter ECG 1 year after the intervention showed sinus rhythm in 18 patients and sinus rhythm with intermittent atrial rhythm in 5 patients. APCs were still observed in 1 patient and seen for the first time in 1 patient; 1 patient and rare ventricular premature complexes. In conclusion, cardiac dysrhythmias on Holter ECG in pediatric patients before and 1 year after transcatheter ASD closure with the ASO device are rare and benign. Regular Holter monitoring seems to be useful in detecting late dysrhythmias.