To evaluate whether transcatheter closure of secundum atrial septal defects (ASD) affected noninvasive electrophysiologic variables in children, we reviewed the preprocedural and 1-year postprocedural electrocardiograms and Holter recordings of 18 consecutive children referred for ASD closure. Patients included in the study were a mean of 5.0 years old (SD 1.1) and weighed a mean of 17.9 kg (SD 4.1). ASDs had a mean diameter of 14.0 mm (SD 2.4) and average shunt ratio (pulmonary-to-systemic flow) of 2.1:1. One year after occluder device placement, 9 children (50%) had detectable residual shunts by transthoracic echocardiograms, but only 2 (11%) had shunts that were felt to be possibly significant. One or more fractured occluder legs were noted by chest roentgenogram in 15 patients (83%). Electrocardiograms at follow-up demonstrated improvement in right ventricular dilation in 4 of 7 patients, right atrial enlargement in 3 of 4 patients, and 1 atrioventricular block in 2 of 3 patients. Holter recordings showed a decreased incidence of accelerated atrial rhythm in 3 of 7 patients, prolonged junctional escape rhythm in 2 of 2 patients, and premature atrial contractions in 2 of 2 patients. No finding correlated with patient age, defect or occluder diameter, occluder leg fracture(s), or residual defects. These improvements in electrophysiologic abnormalities compare favorably with changes seen 1 year after surgical closure. In conclusion, placement of a transcatheter ASD device in children diminishes noninvasive electrophysiologic abnormalities at 1-year follow-up. By relieving hemodynamic stress caused by an ASD in childhood, a transcatheter device may prevent arrhythmia disturbance later in life.