Estimation of right atrial pressure (RAP) using echocardiographic measurement of the inferior vena caval (IVC) size along with its respirophasic variation is commonly performed despite the paucity of data that critically evaluates this technique. In this study, we systematically evaluated echocardiographic imaging of the IVC for estimation of RAP in 102 patients undergoing right heart catheterization. This study established cut-off values using receiver operating characteristic analysis for 8 different IVC parameters and then prospectively tested these parameters for their ability to predict an elevated RAP. The IVC size cutoff with optimum predictive use for RAP above or below 10 mm Hg was 2.0 cm (sensitivity 73% and specificity 85%) and the optimal IVC collapsibility cutoff was 40% (sensitivity 73% and specificity 84%). Traditional classification of RAP into 5-mm Hg ranges based on IVC size and collapsibility performed poorly (43% accurate) and a new classification scheme is proposed.