Transesophageal echocardiography (horizontal sector scan) was performed in 11 patients with secundum atrial septal defect (ASD). In all 11 patients, transesophageal echocardiography presented the definite visualization of the defect and a clear laminar shunt flow that showed its 2 peaks in late systole and late diastole. We estimated the size of ASD and a shunt volume across the defect by using transesophageal echocardiography. The defect size determined by transesophageal echocardiography was correlated with the surgical measurement (horizontal width, r = 0.92, p < 0.001; vertical length, r = 0.85, p < 0.01). A significant high correlation was shown between the shunt volume measured by transesophageal echocardiography and that by Fick's method (r = 0.87, p < 0.01). There was no significant correlation between the pulmonary to systemic flow volume (ratio) and the mean shunt flow velocity across ASD, although a high linear correlation was observed between the pulmonary to systemic flow ratio and the defect size in horizontal direction (r = 0.82, p < 0.01). Transesophageal echocardiography used for diagnosis and quantitative evaluation of ASD could be performed easily and satisfactorily within 10 minutes. Thus, transesophageal echocardiography is a useful method in evaluation of the defect size and the shunt flow volume of ASD. The mean shunt flow velocity was not a reliable index for estimating the shunt flow volume. The defect size might be a valuable determinant of left-to-right shunt volume in ASD. © 1990.