Purpose : 2D echocardiographic evaluation of the severity of aortic stenosis is very difficult because different cut planes are possible, so not the exact portion of the smallest area but just possible cut planes are used in clinical practice. Real time three-dimensional echocardiography (RT3DE) in biplane mode is a novel technique that allows operators to visualize the cardiac anatomy in any desired plane orientation. The purpose of this study is to evaluate the clinical feasibility of assessment of patients with aortic stenosis by RT3DE. Methods : Using aortic valve area (AVA) of several cut planes and distance from aortic annular to aortic valve tip, we can obtain the opening volume of the aortic valve (from aortic annular to aortic valve tip) by two-dimensional Simpson���s volumetric method (software ;General Imaging 3D Quantification). Aortic valve area can be obtained by manually tracing the aortic valve contour on several planes with off-line quantification. Aortic valve area was measured in 23 patients with moderate and severe aortic stenosis using two-dimensional Echocardiography (2D Echo) and the principle of continuity equation (CE). Peak and mean trans-aortic valve pressure gradient were measured using 2D-Doppler Echocardiography. The opening volume of the aortic valve was measured using RT3DE (Philips iE33 platform, using probes S51 and X3) in them. Results: Mean AVA was 0.98 짹 0.36 cm2 determined by 2D Echo and 0.96 짹 0.48 cm2 by 2D Echo-CE and 0.92 짹 0.30 cm2 by RT3DE. Peak and mean trans-aortic pressure gradient were 60.2 짹 39.0 mmHg and 36.2 짹 25.5 mmHg by 2D-Doppler. Mean opening volume of the aortic valve was 1.35 짹 0.58 ml by RT3DE. Opening volume of the aortic valve showed negative correlation with peak and mean trans-aortic pressure gradient (r= -0.55, p= 0.006, and r= -0.56, p= 0.005) and positive correlation with AVA by using 2D Echo and 2D Echo-CE (r= 0.48, p= 0.019, and r= 0.59 p= 0.003). Conclusions: Good correlation was found between opening volume of the aortic valve determined by RT3DE and trans-aortic pressure gradient. Opening volume of the aortic valve determined by RT3DE in biplane mode can be a good parameter for the evaluation of the severity of aortic stenosis. Keyword : Realtime Three-Dimensional Echocardiography, Aortic stenosis