We report 100 cases of mitral commissurotomy in children, 15-years-old or less, suffering from rheumatic mitral stenosis. Mitral stenosis in children is characterised by the severity of functional impairment and the considerable radiological, electocardiological and haemodynamic changes, with pulmonary arterial hypertension which is always present and often well marked. The early results of mitral commissurotomy are very satisfactory with clinical improvement and a low mortality. But, in the long term, we have seen progressive deterioration in the clinical state of these patients resulting from re-stenosis or from the progression of another valve lesion. Three problems have been discussed. First, the progression of the rheumatic process which seems to account to a large extent for the late failures of mitral commissurotomy in children. Secondly, the problem of tricuspid insufficiency, which is often associated with mitral stenosis in childhood and which usually disappears during the post-operative period. Finally, the problem of pre-capillary pulmonary arterial hypertension which always showed a tendency towards regression.