Background: The effect of ASD shape on ASD size measurements remains unclear. This study was performed to assess the relationships between stretched balloon diameter (SBD) and diameters measured by 2-dimensional (2D) and 3-dimensional (3D) transesophageal echocardiography (TEE), and to evaluate the effect of atrial septal defect (ASD) shape on these relationships in patients with secundum ASD. Methods: We prospectively enrolled 107 consecutive patients who underwent transcatheter closure of ASD. SBDs and ASD diameters on 2D and 3D TEE were measured. The circular index of ASD was defined as the ratio of maximal to minimal diameter on 3D TEE image. All patients were clinically followed up for a median of 1.3 months after the procedure. Results: The correlations between measurements were significantly better in patients with round (circular index < 1.5) than with ovoid ASD (Figure). The differences between SBDs and maximal diameters measured by 3D TEE were significantly smaller in patients with large ovoid ASDs than in other patients (Table). The differences between the size of finally implanted devices and the maximal diameters measured by 3D TEE were smaller in patients with large ovoid than with round ASDs. There were no major complications, including device embolization, during follow-up. Conclusions: The relationships between SBD and maximal diameters measured by 2D and 3D TEE are influenced by ASD shape. All parameters including ASD shape should be considered in selecting device size for transcatheter ASD closure.