A comprehensive study of more than 1,700 children and adolescents with rheumatic fever has been in progress since 1920 at the House of the Good Samaritan in Boston. This large group includes 81 patients who have acquired the physical signs of so-called pure mitral stenosis. These physical signs consist of (1) a low-pitched (often coarse) murmur at or near the cardiac apex, which begins in mid or late diastole and ends with crescendo in (2) an abrupt, slapping first sound. A corresponding thrill and shock are commonly associated with the more advanced lesions, and accentuation of the pulmonary second sound is usually present. For our purpose in this special study we have excluded a considerable number of patients with undoubted mitral stenosis who, in addition to the physical signs mentioned, had a blowing systolic murmur at the cardiac apex, characteristic of associated mitral regurgitation. We realize that in many of