Background: Accuracy of planimetry of the mitral valve area (MVA) in patients with mitral stenosis (MS) by 2D echocardiography (2DE) depends on operator skill for the acquisition of the image of mitral valve (MV) opening during diastole. This study was done to determine the feasibility and reproducibility of the real-time 3D echocardiography (RT3DE) for the estimation of MVA in comparison with 2DE. Methods: In 30 patients with MS (m : f = 8 : 22, mean age: 51 짹 11 yrs), volumetric data set was acquired from parasternal view. MVA was measured off-line using 3D computer software (TomTec, Co.) by two independent observers at different times. The same images were also analyzed on a different day by one of these same observers. From two orthogonal longitudinal planes that were vertical to the tip of MV opening, cross sectional plane at the level of the tip of MV opening was acquired during mid diastole for the planimetry of MVA. MVA was also determined by 2DE planimetry and Doppler pressure half time (PHT). Results: There was no significant difference between the MVA measured by 2DE and PHT (1.35짹0.57 vs 1.36짹0.52 cm², p > 0.05). The MVA by RT3DE was significantly smaller than that by 2DE (1.19짹0.53 vs 1.35짹0.57 cm², p < 0.05). However, it showed close correlations with those by 2DE and PHT (r = 0.96 and r = 0.88, respectively). The correlation coefficient obtained between the two observers was 0.96 (y = 0.1007 + 0.92x; standard error of the estimate = 0.16). The variability of intraobserver and interobserver measurements was not significantly different (p = 0.73 and p = 0.97, respectively). Conclusion: Using RT3DE, we obtained smaller MVA than that by 2DE. Planimetry with RT3DE seems to be a feasible method for the measurement of MVA in MS with good reproducibility providing smaller measurements than those with 2DE, which hopefully would be closer to true value.