||Clinical and/or hemodynamic studies were carried out on 39 patients with mitral stenosis, before and after surgery. Criteria have been established for more precise diagnosis and an attempt has been made to evaluate objectively the results of mitral surgery. To determine the significance of mitral stenosis, in the presence of mitral insufficiency, no single criterion was infallible. The contour of pulmonary artery "wedge" pressure tracing was sometimes helpful; otherwise the hemodynamic data were of not value. In general, the following criteria were found helpful in distinguishing predominance of mitral stenosis or insufficiency, in this series (Mitral Stenosis 1. opening snap, 2. accentuated first heart sound at apex, 3. absence of loud systolic murmur at apex, 4. normal sized heart, and 5. electrocardiographic evidence of mitral P waves, incomplete right bundle branch block or right ventricular hypertrophy; Mitral Insufficiency 1. loud apical systolic murmur, 2. enlarged heart with left ventricular enlargement, and 3 tall V waves in PA "wedge" pressure tracing). Hemodynamic data were found to be very helpful in evaluating the degree of incapacitation of those patients with mitral stenosis in whom the subjective complaints were out of proportion the objective clinical findings. It is concluded that the presence of normal hemodynamics is a definite contraindication to mitral surgery in spite of any clinical findings. The results of surgery were independent of the presence of a slight amount of mitral regurgitation, the initial value of pulmonary resistance, or the microscopic finding in the auricular biopsy. Objective evidence of improvement was found by electrocardiogram, x-ray and hemodynamic data in 70% of the patients with pure of predominant mitral stenosis who improved subjectively. It is stressed that a conclusion concerning the results of surgery in any given case should be reserve until objective data are available to substantiate the subjective and clinical improvement.