The article examines the contribution of real-time three-dimensional transesophageal echocardiography (3D TEE) in the assessment of ostium secundum atrial septal defects (ASD) in adults. 3D TEE provides a precise and comprehensive view of cardiac anatomy, enabling the evaluation of size, shape, and border quality of the ASD. In a series of 27 cases, 3D TEE demonstrated a strong correlation between 3D-measured ASD parameters and the size of the closure device required. This technique is crucial for accurate pre-interventional evaluation, enhancing the success and safety of percutaneous or surgical interventions for ostium secundum ASD. Three-dimensional transesophageal echocardiography represents a significant advancement in cardiovascular imaging for complex structural heart abnormalities.