The findings in and experiences with 19 consecutive patients subjected to open mitral valve surgery are described. All patients underwent a right-heart catheterization. In order to exclude multivalvular heart disease, a left-heart catheterization was performed in 10 patients and angiographic studies of the aortic valve area in 12. Pulmonary function studies were performed on 11 patients. The FEV (0.75 sec.) and the MMFR were found to correlate well with existing pulmonary reserves.Fourteen of the 19 patients subjected to open mitral valve surgery survived the operation and have been followed up for three months to 2(1/2) years. Four patients had a mitral valvuloplasty. Three of these four have deteriorated and will require a valvular replacement. Ten of 15 patients subjected to a mitral valve replacement are alive; five of these 10 have had signs and symptoms indicating peripheral embolization. Mitral valvuloplasty is preferable to a valvular replacement as far as embolic complications are concerned, while valvular replacements result in a more perfect and lasting hemodynamic repair. The high incidence of peripheral embolization following valvular replacements focuses attention on the need for improved mitral valve prostheses.