A new balloon catheter was developed which allows mitral commissurotomy without thoracotomy. The procedure has been successful in five of the six patients with mitral stenosis so treated. In the remaining patient, the procedure could not be performed because of technical difficulties. The balloon is reinforced with a nylon micromesh and its shape changes in three stages, depending on the extent of inflation. It is inserted from the saphenous vein into the mitral orifice transseptally, fixed across the mitral orifice with partial inflation, and finally inflated to full its extent, separating the fused commissures by its expansile force. After the procedure, catheterization revealed a significant reduction in the mean diastolic pressure gradient across the mitral valve without resultant mitral regurgitation in each patient. Two-dimensional echocardiograms showed a marked to moderate degree of dilatation of the mitral orifice in each patient. All five patients are well with remarkable clinical improvements 2 to 16 months after the procedure.