The study aimed to investigate the intermediate-term effects of transcatheter atrial septal defect (ASD) closure on cardiac remodeling in children and adult patients. Between December 2003 and February 2009, 117 patients (48 males, 50 adults) underwent transcatheter ASD closure with the Amplatzer septal occluder (ASO). The mean age of the patients was 15 years, and the mean follow-up period was 25.9 ± 12.4 months. New York Heart Association (NYHA) class, electrocardiographic parameters, and transthoracic echocardiographic (TTE) examination were evaluated before the ASD closure, then 1 day, 1 month, 6 months, 12 months, and yearly afterward. Transcatheter ASD closure was successfully performed for 112 (96%) of the 117 patients. The mean ASD diameter measured by transesophageal echocardiography (TEE) was 14.0 ± 4.2 mm, and the mean diameter stretched with a sizing balloon was 16.6 ± 4.8 mm. The mean size of the implanted device was 18.6 ± 4.9 mm. The Qp/Qs ratio was 2.2 ± 0.8. The mean systolic pulmonary artery pressure was 40 ± 10 mmHg. At the end of the mean follow-up period of 2 years, the indexed right ventricular (RV) end-diastolic diameter had decreased from 36 ± 5 to 30 ± 5 mm/m² (p = 0.005), and the indexed left ventricular (LV) end-diastolic diameter had increased from 33 ± 5 to 37 ± 6 mm/m² (p = 0.001), resulting in an RV/LV ratio decreased from 1.1 ± 0.2 to 0.8 ± 0.2 (p = 0.001). The New York Heart Association (NYHA) functional capacity of the patients was improved significantly 24 months after ASD closure (1.9 ± 0.5 to 1.3 ± 0.5; p = 0.001). At the 2-year follow up electrocardiographic examination, the P maximum had decreased from 128 ± 15 to 102 ± 12 ms (p = 0.001), the P dispersion had decreased from 48 ± 11 to 36 ± 9 ms (p = 0.001), and the QT dispersion had decreased from 66 ± 11 to 54 ± 8 ms (p = 0.001). Five of six patients experienced resolution of their preclosure arrhythmias, whereas the remaining patient continued to have paroxysmal atrial fibrillation. A new arrhythmia (supraventricular tachycardia) developed in one patient and was well controlled medically. Transcatheter ASD closure leads to a significant improvement in clinical status and heart cavity dimensions in adults and children, as shown by intermediate-term follow-up evaluation. Transcatheter ASD closure can reverse electrical and mechanical changes in atrial myocardium, resulting in a subsequent reduction in P maximum and P dispersion times.