We attempted percutaneous transcatheter-balloon mitral commissurotomy in eight children and young adults (9 to 23 years of age) with rheumatic mitral stenosis. The atrial septum was traversed by needle puncture, and an 8-mm angioplasty balloon was advanced over a guide wire. The atrial septal perforation was then dilated to allow passage of the valvuloplasty balloon catheter (18 to 25 mm) across the mitral annulus. Inflation of the transmitral balloon decreased the end-diastolic transmitral gradient temporarily in all patients (from 21.2 +/- 4.0 mm Hg [mean +/- S.D.] to 10.1 +/- 5.5 mm Hg; P less than 0.001). The immediate decrease in the gradient was associated with increases in cardiac output (from 3.8 +/- 1.0 to 4.9 +/- 1.3 liters per minute per square meter of body-surface area; P less than 0.01) and in the calculated mitral-valve-area index (from 0.73 +/- 0.29 to 1.34 +/- 0.32 cm2 per square meter; P less than 0.001). Murmur intensity diminished immediately after commissurotomy in all patients. The greatest reduction in pressure gradient (76 to 95 per cent) occurred when the largest balloon (inflated diameter, 25 mm) was used in the smallest patients (0.9 to 1.2 m2). The balloon commissurotomy produced minimal mitral regurgitation in only one child. Follow-up catheterization (at two to eight weeks) demonstrated persistence of hemodynamic improvement with evidence of partial restenosis in one patient. These early results indicate that balloon mitral commissurotomy can be a safe and effective treatment for children and young adults with rheumatic mitral stenosis.