SURGICAL relief of mitral obstruction may now be said to have become an established practice. In sharp contrast to the insurmountable difficulties which beset the attempts to apply this form of therapy in the 1920's, current procedures have the advantage of advances in surgical technique, in anesthetization, in preoperative and postoperative care, and in the control of infection with chemotherapy and antibiotics. Furthermore, it is now possible, with techniques of cardiac catheterization and tests of circulatory and pulmonary function, to appraise, in more or less quantitative terms, the physiological changes consequent upon the obstruction at the mitral valve. Finally, changes in pressure in the pulmonary circulation and even in the left auricle may also be checked at the time of operation. Not only have such measurements served to confirm deductions, based on symptoms and physical signs, with regard to the dynamic consequences of mitral stenosis, but they have demonstrated the