Balloon dilatation of the mitral valve was performed, trans-septum via the femoral vein or retrogradely via the femoral artery, in 20 patients, aged 31-65 years, with post-rheumatic mitral stenosis. Haemodynamically insignificant mitral regurgitation occurred in two patients. In one patient a permanent atrial septal defect was produced by the transseptal puncture. In a fourth patient severe mitral regurgitation occurred which required operation. There were no other serious complications. No dilatation was possible in three patients for technical reasons. Successful dilatation was achieved in 16 of the 20 patients. The end-diastolic transvalvular pressure gradient was reduced from 12 +/- 5 to 4 +/- 3 mm Hg. At follow-up, after 3-6 months (12 patients) or after 1-1.5 years (5 patients), in one instant after a second dilatation, a lasting good result was confirmed, the mean end-diastolic gradient being 4 +/- 3 mm Hg.