Aims The aim of this study was to assess the value of two-dimensional (2D) transthoracic simultaneous multiplane imaging (SMPI) in the evaluation of suitability for percutaneous atrial septal secundum defect (ASD) closure compared with the golden standard 2D transoesophageal echocardiography (TEE). Methods and results Twenty-nine patients with an ASD underwent both SMPI and TEE. Ten patients (34%) were male (age 41 +/- 18 years, range 20-74). SMPI assessment of ASD size and rims included xPlane and I-rotate modes. Rims were defined as suitable for ASD percutaneous closure using a cut-off value of 5 mm. There were no significant differences between SMPI in xPlane mode and TEE regarding the sizes of the anterior-posterior dimension (13.7 +/- 4.5 vs. 14.5 +/- 5.2 mm) and superior-inferior dimension (13.5 +/- 3.9 vs. 14.1 +/- 5.0 mm, respectively). Agreement for the aortic, atrioventricular, inferior, right upper pulmonary vein, and superior rims was 100, 100, 100, 96, and 96%, respectively. Conclusion The SMPI technique can reliably assess the dimensions and rim size of a secundum ASD for pre-interventional selection when compared with TEE and has thus the potential to replace TEE.