The RELIEF of obstructing lesions of the mitral valve by mitral commissurotomy is now a widely accepted and extremely successful procedure. Sufficient time has elapsed since this procedure was first initiated to permit a valid appraisal of the over-all results. Although symptomatic results are important, the accumulation of objective data, when possible, adds greatly to our knowledge concerning the effect of an operative procedure. Such data are available in the form of hemodynamic studies carried out before operation, during operation, and at intervals after operation on patients undergoing mitral commissurotomy. We believe that the results of these hemodynamic studies present ample explanation for the marked symptomatic improvement that fully three quarters of these patients experience following operation. OVER-ALL EXPERIENCE We are presenting here our experience in 131 cases of mitral stenosis in which mitral commissurotomy was performed prior to April, 1954. Surgical incision of the commissures has been employed in