Percutaneous balloon valvoplasty of the mitral valve was performed in 126 patients (24 men, 102 women; mean age 55.0 +/- 12.1 years) with mitral stenosis. The mean transmitral valve gradient fell from 12.4 +/- 6.1 to 6.0 +/- 3.2 mmHg, while the valve opening area increased from 1.0 +/- 0.2 to 1.55 +/- 0.3 cm2. After percutaneous balloon valvoplasty 36 patients still had no mitral regurgitation, while the grade of mitral regurgitation remained the same in 47 (grade I: n = 35; grade I: n = 12). Mitral regurgitation, previously not present, occurred in 25 patients, but was severe in only three (grade III: n = 1; grade IV: n = 2). Previously present mitral regurgitation increased in 18 of 65 patients, in four to grade III, in one to grade IV. In three patients acute grade IV mitral regurgitation resulted from a tear in a leaflet of a fibrotic valve which was not or only slightly calcified, requiring emergency surgery. Followup observations over 16.9 (1-60) months showed no change in most patients, but three developed mitral regurgitation. The latter underwent elective surgery, as did one patient with acute mitral regurgitation. Thus a total of seven patients (5.6%) required surgery for mitral regurgitation after percutaneous balloon valvoplasty.