In this report we present 12 adult patients in whom surgical or percutaneous intervention was considered for repair of atrial septal defect (ASD). Location, size, and surrounding atrial anatomy of the ASD were assessed prior to intervention in all patients with standard and live three-dimensional transthoracic echocardiography (3D TTE). In the four patients in whom intraoperative three-dimensional transesophageal echocardiographic reconstruction (3D TEE) was done, 3D TTE measurements of maximum dimension, maximum circumference, and maximum area of ASD agreed well with 3D TEE. In the seven patients who underwent transcatheter closure device insertion, live 3D TTE measurements of maximum dimension, maximum circumference, and maximum area of ASD agreed well with the sizing balloon. Additionally, since the sizing balloon measures a stretched diameter and area, a live 3D TTE stretched ASD diameter and area (derived from the actual live 3D TTE maximum circumference) were calculated and demonstrated improved agreement with the sizing balloon measurements. In all patients, ≥5 mm of atrial tissue was visualized surrounding the ASD. Further, with the addition of contrast enhancement, characterization of a small patent foramen ovale (<5 mm) was possible in one patient. Live 3D TTE accurately defined ASD location, size, and surrounding atrial anatomy in all patients studied by us. ASD characterization by live 3D TTE agreed well with 3D TEE and sizing balloon measurements.