The purpose of this study was to evaluate morphological and functional MRI of atrial septal defects (ASD) before and after interventional occlusion by the Amplatzer Septal Occluder (AOC) in comparison to trans-oesophageal echocardiography (TEE), invasive balloon measurement (IVBM) and cardiac catheterisation (QCC). Sixty patients with an ASD type II were enrolled. They underwent TEE, IVBM, QCC and MRI at 1.5T. Cine gradient echo, steady-state free precession sequences and a gradient echo phase contrast sequence were used. In MRI, pulmonary-to-systemic flow ratio (Qp/Qs) was calculated and compared with the QCC Qp/Qs ratio. Qp/Qs ratio in baseline MRI examination was 1.56 +/- 0.29 (range: 1.05-2.2) and in QCC 1.71 +/- 0.30 (range: 1.2-2.4) with a significant correlation (R = 0.65, P < 0.01). Defect size on MRI was 15.3 +/- 7.4 mm (range: 3-30 mm), in TEE 14.3 +/- 4.9 mm (range: 4-24 mm), and the balloon stretched diameter in IVBM was 23.4 +/- 4.2 mm (range: 14-32 mm). Correlation between defect size in MRI vs. TEE was R = 0.67 (P < 0.01) and MRI vs. IVBM was R = 0.77 (P < 0.01). Right ventricular volumes decreased after intervention. MRI is an accurate noninvasive test for diagnosis, planning and follow-up after interventional ASD occlusion using an AOC.