Occlusion of the atrial septal defects in the oval fossa by interventional catheterization has progressed, but still has limitations. Three-dimensional (3D) echocardiography can provide unique views unavailable by cross-sectional imaging. The objective of this study was to define the clinical application of 3D echocardiography in the assessment and monitoring of transcatheter occlusion of atrial septal defects. Three-dimensional echocardiography was attempted prior to occlusion of atrial septal defects in 41 patients (median age 8.6 years). Serial cross-sectional images were acquired by multiplane transoesophageal echocardiography and displayed by means of computer reconstruction. Dynamic 3D echocardiographic images of defects in the oval fossa were obtained in 40 of 41 patients (98%). Volume-rendering demonstrated the anterosuperior rim in 36 (90%) and the inferoposterior rim in 24 (60%), but failed to reveal small additional fenestrations in six. Sizes measured by 3D echocardiography were significantly larger than those provided by cross-sectional transoesophageal echocardiography (p =;0.007), but differed little from those obtained with balloon sizing (p =; 0.6). After occlusion, 3D echocardiography showed positions of all arms of the device in 20 of 24 cases. Location of any protruding arms, or residual defects, were also clearly revealed.Three-dimensional images obtained in 12 patients during deployment of the double-umbrella device were useful in monitoring its position (single-frame) and for explaining the mechanism of protrusion. Current 3D echocardiography provides clinically relevant information for selection of patients for closure of atrial septal defects by interventional catheterization and when monitoring during implantation. Information obtained by this technique can clarify the mechanism of deployment of the device and closure of the defect, therefore influencing outcomes.