Cardiac arrhythmias and right chamber enlargement are well known long-term sequelae of atrial septal defect (ASD). Surgical ASD closure relieves patient symptoms but often fails to revert cardiac volume overload findings. Transcatheter ASD closure might be an attractive alternative to surgery, also because of the possibility to study the amount and time-course of the electro-geometric modifications following shunt disappearance. Between March 2000 and December 2002, 24 patients (age 22.7 +16.8 years) underwent percutaneous closure of large ASD (stretched diameter >20 mm and/or QP/QS ratio >1.5:1). ASD closure was performed with the Amplatzer Septal Occluder device (mean 25 ± 7 mm), achieving a complete occlusion in all patients at 1 month. In 6 patients, right ventricular (RV) monophasic action potential was recorded during the procedure. All patients underwent standard 12-lead electrocardiography and transthoracic echocardiography before and at 24 hours and 1 month after ASD closure. After the procedure, monophasic action potential length increased from 359 ± 27 to 372 ± 27 ms (p <0.0001). At 1 month, QT dispersion decreased from 54 ± 25 to 41 ± 17 ms (p <0.05), RV diastolic diameter decreased from 42 ± 6 to 34 ± 5 mm (p <0.00001), and left ventricular (LV) diastolic diameter increased from 39 ± 5 to 44 ± 5 mm (p <0.0001), resulting in a decrease in the RV/LV ratio from 1.11 ± 0.22 to 0.79 ± 0.11 (−28.8%, p <0.00001). Electrocardiographic changes, as well as the amount and time-course of RV overload relief, did not significantly differ between pediatric (<16 years of age; n = 11) and adult patients (n = 13). In conclusion, regardless of age at procedure, percutaneous ASD closure results in early striking electrical and geometric cardiac changes that may be beneficial during long-term follow-up.