The diastolic rumbling murmur of mitral stenosis (MS) may be attenuated in the presence of low cardiac output, right ventricular enlargement, Lutembacher's syndrome, pulmonary emphysema, and obesity. In this report we would like to stress that the presence of tricuspid stenosis (TS) is an additional significant cause of silent MS. The clinical material consisted of 73 patients with rheumatic TS who had undergone cardiac surgery. Five of these cases had clinical findings of TS without auscultatory findings of MS. They were found to have severe MS at the time of operation and to require mitral valve surgery. At cardiac catheterization the mean diastolic gradient (MDG) across the mitral valve (MV) was less than 3 mmHg and pulmonary arterial systolic pressure was 29-42 mmHg. The MDG across the tricuspid valve was 6-17 mmHg. In conclusion, TS can mask clinical and hemodynamic findings of MS. The reason for this is the mechanical barrier imposed by TS proximal to the MV.