Summary:The ability of two-dimensional echocardiography (2DE) to quantitate the atrial septal defect size and left-to-right shunt magnitude was examined in 75 adult patients with simple ostium secundum atrial septal defect (ASD) with left-to-right shunts of 19-92% of systemic flow as determined by oximetry. The ASD was visualized in 7 1 of 75 (95 %) patients utilizing subcostal 2DE, and the end-systolic atrial septal defect diameters in subcostal 2DE (ASDe) were measured. The maximal diameters of ASD measured during operation (ASDop) were obtained in 45 of these patients, who then underwent surgical ASD repair. The correlation between ASDe and ASDop was high (r=0.91, p2.0 cm and RVD/LVD > 1.1 had a left-to-right shunt > 40%. In conclusion: (1) ostium secundum atrial septal defect size can be reliably