Abstract Surgical correction of mitral stenosis has become an accepted procedure in selected cases. This has made it important to diagnose preoperatively in such cases the degree of competence of the mitral valve as well as the degree of stenosis. At present our tools for diagnosing significant mitral insufficiency in patients with rheumatic mitral valve disease are imperfect. Thus, many series of mitral commissurotomies report cases in which cardiotomy revealed unexpected predominant mitral insufficiency. 1–9 The present paper reports some electrocardiographic features of significant mitral insufficiency which came to our attention during the first 126 operations for mitral valvulotomy at The New York Hospital.