In two female patients (59 and 30 years old) with mitral stenosis, a successful valvuloplasty was carried out with a balloon catheter. After transseptal puncture with determination of the pressure gradient between the left auricle and the left ventricle, the atrial septum was initially dilated with a small balloon and a MediTech balloon catheter with a diameter of 25 mm was pushed forward into the region of the mitral valve via a guide wire. On filling of the balloon, the marked indentation at 3-4 atü hyperbaric pressure initially present could be eliminated. The pressure gradient fell from 8 mm Hg to 2 mm Hg in the first patient and from 12 mm Hg to 3 mm Hg in the second patient. The interventions were without complications. There was a marked clinical improvement with increase of physical exercise tolerance and decrease of dyspnoea in both patients. A mitral valve insufficiency did not occur. The new technique may possibly constitute an alternative to surgical commissurotomy.