Phonocardiograms recorded before operation in the usual auscultation points were compared with the findings during operation in 223 patients and the results of autopsy of 12 persons who had undergone mitral commissurotomy. A systolic murmur at the apex of the heart had been recorded in 184 patients and in 9 persons who had died. In 87 patients it was associated with regurgitation. In 97 patients and in 9 who had died a pure mitral stenosis was revealed on operation. Besides the possible regurgitation as the result of coase deformity of the mitral valve and infravalvular apparatus, the causes of the systolic murmur may be relative incompetence of the tricuspid valve, functional stenosis of the pulmonary artery and concomitant aortic stenosis. In 10 cases the cause of the systolic murmur was not disclosed. The question of its origin calls for further study.