Background: Recently introduced new echocardiographic techniques, single-beat full-volume capture real-time three-dimensional echocardiography (RT3DE) and auto-contouring algorithm have allowed us to get more useful and convenient echocardiographic images. In congenital heart disease such as atrioventricular septal defect, left ventricular morphology (LV) is different from normal LV. The aim of this study was to evaluate the feasibility and accuracy of LV volume measurements by RT3DE using the single-beat full-volume capture technique. Method: Ten patients with atrioventricular septal defect six patients with ventricular septal defect were enrolled (age 23짹5 years, ranged from 16 to 30 years) and performed RT3DE using the single-beat full-volume capture. LV end-systolic volume, end-diastolic volume, and ejection fraction were measured using an auto-contouring algorithm from data acquired on RT3DE, in addition to conventional echocardiography. Results: Volume measurements on RT3DE with single-beat full-volume capture were feasible in all patients. Both interobserver and intraobserver variability was excellent agreement. LV morphology in AVSD patients is more spherical and eccentric (sphericity index 0.70짹0.08 vs. 0.53짹0.01; p <0.001, eccentricity index 0.76짹0.04 vs. 0.97짹0.03; p <0.001). In comparison to simple VSD patients (r =0.88, P=0.02), 2D volumetric and M-mode measurements in AVSD patients are less correlated with RT3DE measurements (r=0.646, p =0.44) in Pearson's correlation analysis. However, the rate of successful auto-contouring algorithm was lower in AVSD patients (6/10 vs. 1/6) Conclusion: RT3DE using single-beat full-volume capture is an easy and more reliable technique to assess LV volume and systolic function in AVSD patients, compared to conventional echocardiographic methods. Further validated studies & high frequency applications for small children are needed.