Cross-sectional echocardiograms of the mitral valve orifice were recorded in 37 patients with mitral stenosis. Twenty-seven had pure mitral stenosis, and 10 had associated mitral regurgitation. Mitral valve area in patients with pure mitral stenosis measured from cross-sectional echocardiography was highly correlated (r = 0.89) with that calculated with the Gorlin formula using the pressure gradient and Fick cardiac output. With mitral regurgitation, mitral valve area by cross-sectional echocardiography correlated well (r = 0.90) with that calculated from the pressure gradient and cineangiographic stroke output. In two cases, direct pathologic measurements of mitral valve area agreed exactly with the cross-sectional echocardiographic measurement. Correlation between the mitral E-F slope and mitral valve area by cross-sectional echocardiography (r = 0.56) and catheterization (r = 0.49) was less reliable. Cross-sectional echocardiographic measurement of the mitral valve area correlates well with catheterization in patients with pure mitral stenosis and those with associated regurgitation.