Forty-three patients with suspected restenosis of the mitral valve are reported. Forty of these occurred in a series of 672 mitral commissurotomies. A significant degree of recurrent stenosis was found in 40 of the 43 cases at the time of the second operation. The clinical features and findings in these patients are discussed, as well as the probable etiologic factors. Left heart catheterization is indicated in many of these cases to define precisely the hemodynamic status, particularly in those individuals with suspected concomitant mitral incompetency or myocardial disease. Open-heart surgery should significantly improve the results of a repeat mitral commissurotomy.