The efficacy of balloon dilatation of the mitral valve by a bifoil (2 x 19 mm) or trefoil (3 x 15 mm) catheter (single catheter technique) was assessed in 53 patients (mean age 28) with mitral stenosis, most of whom were women. The procedure was unsuccessful in three patients. After balloon dilatation the left atrial pressure decreased from 22 mm Hg to 13 mm Hg and the mitral valve gradient from 12 mm Hg to 4 mm Hg. The mitral valve area increased from 0.7 cm2 to 2.1 cm2. Exercise time on the standard Bruce protocol increased from 3.9 minutes to 7.2 minutes. In 22 (44%) patients mitral regurgitation developed or the grade of regurgitation increased. Left to right shunts with pulmonary to systemic flow ratios greater than 1:5 were detected in four patients. Transient cerebrovascular episodes developed in two patients. One patient died after emergency valve replacement for severe mitral regurgitation. Balloon dilatation of the mitral valve by the single catheter technique with the bifoil or trefoil catheters is an effective treatment for patients with mitral stenosis. Mild mitral regurgitation is a frequent complication of the procedure.