Objective: To characterise prospectively by magnetic resonance imaging (MRI) changes in right ventricular (RV) volume, function, and mass after transcatheter closure of atrial septal defects (ASDs) and to evaluate the course of pulmonary pressure and functional class criteria. Methods: In 20 patients with secundum-type ASD and dilated RV diameter, MRI was performed to quantify RV end diastolic (RVEDV) and end systolic volumes (RVESV), RV mass, tricuspid annular diameter, and RV ejection fraction before and 6 and 12 months after transcatheter closure of the ASD. RV systolic pressure was measured during follow up by transthoracic echocardiography. Results: Functional class improved in the majority of patients after ASD closure. RVESV (from 81 (18) ml/m ² to 53 (15) ml/m², p < 0.001), RVEDV (from 127 (17) ml/m² to 99 (18) ml/m², p < 0.001), and RV mass (from 79 (10) g to 63 (8) g, p < 0.01) decreased significantly during follow up, although tricuspid annular diameter did not. RV ejection fraction improved (by 9% compared with baseline, p < 0.05) and RV systolic pressure decreased significantly (from 33 (8) mm Hg to 24 (6) mm Hg, p < 0.001) after closure. Conclusion: MRI studies showed significant improvement of RV volumes, mass, and function after transcatheter closure of ASDs. Restoration of the RV leads to decreased pulmonary pressure resulting in a better functional class in the majority of patients.