A new echocardiographic index of mitral valve diastolic closure, based on the rate of diastolic apposition of the anterior and posterior mitral leaflet echoes, was measured in 40 patients with mitral stenosis. This mitral valve closure index correlated highly significantly with the mitral valve orifice area (calculated from the Gorlin formula) (r = 0-87). Correlation between the diastolic closure rate (based) on the EF slope) and the calculated valve area was poor (r = 0-37). It is proposed that the mitral valve closure index excludes movement extraneous to the mitral apparatus and expresses the actual rate of valve closure, thus avoiding some of the factors known to contribute to the poor specificity of the diastolic closure rate. The better correlation of the mitral valve closure index with the calculated valve orifice area makes it possible to assess the severity of mitral stenosis by echocardiogram with greater accuracy and confidence.