IT is common for patients with severe disability from chronic rheumatic mitral valvular disease to present with the apical pansystolic murmur of mitral regurgitation, and the planning of the surgical treatment depends upon the diagnosis of the lesion. Valvotomy may be carried out when the regurgitation is trivial and the obstruction severe, and the valve may be repaired in cases of predominant regurgitation if the aortic cusp is mobile. Prosthetic replacement of the valve may be the only possible treatment where immobility of the cusps and chorcde is associated with a mixture of obstruction and regurgitation. It is reasonable to look for diagnostic principles in the relationships between the clinical and physiological abnormalities and the surgeon's findings at operation, but the author found little of value in attempts to relate the surgeon's opinion of the regurgitation to the systolic murmur, the systolic thrust of the apex of the left ventricle, the systolic expansion of the left atrium, the cardiographic and radiological evidence of left ventricular hypertrophy, the left atrial 'v' wave and 'x' and 'y' troughs, or to estimates of regurgitation made from indicator-dilution curves. Then, in I957, McDonald and his colleagues made the point that a relationship must exist between the size of the mitral orifice and the severity of the regurgitation: the patients with the smallest orifices must be suffering from severe obstruction and little regurgitation, and the patients with the largest orifices must be suffering from severe regurgitation with little or no obstruction (McDonald, Dealy, Rabinowitz and Dexter, 1957). In practice it is easier to estimate the long diameter of the mitral orifice than its area (Goodwin, Hunter, Cleland, Davies and Steiner, 1955). Anatomically, the mitral orifices encountered have measured anything from under i cm. to more than 5 cm. in their long diameter. Physiologically, however, the findings have not placed the patients in a spectrum, or continuous series extending from one extreme of regurgitation to the other, but divided them into three distinct groups, namely, the group with the syndrome of obstruction, the group with the syndrome of incompetence, and the group with the syndrome of the rigid valve. The purpose of this paper is to consider the physiological basis and the therapeutic implications ofthe grouping. It is to be emphasized that patients with severe disability and the apical murmur of mitral regurgitation are the subject of the paper, and not those with pure obstruction or slight disability, nor those with the complication of noteworthy ischkmic, hypertensive, or aortic valvular heart disease.