Background: Left ventricular dyssynchrony (LVD) has become a therapeutic target using biventricular pacing in selected heart failure patients. Recent advances in 3-dimensional echocardiography (3DE) and automated border detection (ABD) techniques have simplified quantification of global LVD. However, the accuracy and reproducibility of 3DE with ABD for measuring LVD have not been evaluated before. Methods: The 3DE data for 50 patients were analyzed offline by 2 independent observers. Systolic dyssynchrony index (SDI) was determined by 3-dimensional ABD (3ABD) and 2-dimensional ABD (2ABD) with different imaging planes (2, 4, 6, 8, 12, 16 and 32). Data for 2-, 4-, 6-, 8-, 12-, 16- and 32-plane assessments were compared to those for 3ABD approach and repeat measurements. Results: Biplane, 4-plane, and 6-plane assessment tended to overestimate SDI. As measured by 8-, 12-, 16- and 32-plane methods, SDI exhibited no significant difference and excellent correlation. The 3DE with 3ABD produced lower intra-observer and inter-observer variability (intra-observer: 7.9% and 14.8%; inter-observer: 9.8% and 16.4% for 3ABD and 8-plane, respectively). Total time for 3ABD approach (3.1±0.6 min) was shorter than conventional 8-plane (14.4±1.2 min, P＜0.001) approach. Conclusion: Accurate quantification of SDI can be obtained from 3DE using 3ABD or 2ABD with as few as 8 planes. To assess SDI, 3DE using 3ABD exhibited better reproducibility than 3DE using 2ABD. Therefore, 3DE assessment using 3ABD is the preferred approach when evaluating global LVD is necessary. Further prospective study is necessory to demonstrate whether this SDI by 3ABD is a predictor of cardiac resynchronization therapy outcome or not.