The immediate outcome of the first 150 patients (Group 1) and the last 161 patients (Group 2) who underwent percutaneous mitral balloon valvuloplasty was compared. There was no difference between the two groups in age, gender, New York Heart Association functional class, presence of calcification, atrial fibrillation, degree of mitral regurgitation, mean pulmonary artery pressure, left atrial pressure, cardiac output, pulmonary vascular resistance, mitral valve gradient and mitral valve area. Fewer patients in Group 1 than Group 2 had an echocardiographic score less than or equal to 8 (62% versus 69%, respectively, p = 0.02). The atrial septum was dilated with an 8 mm balloon in 74% of patients in Group 1 and with a 5 mm balloon in all patients in Group 2. Ratio of effective balloon dilating area to body surface area was larger in Group 1 than in Group 2 (4.05 +/- 0.07 versus 3.7 +/- 0.03 cm2/m2, p = 0.0001). A good result (mitral valve area greater than or equal to 1.5 cm2) was obtained in 77% and 75% in Groups 1 and 2, respectively (p = NS). After percutaneous mitral valvuloplasty, a greater than or equal to 2 grade increase in mitral regurgitation was noted in 12% of Group 1 and 6% of Group 2 (p = 0.02) and a left to right shunt was detected in 22% of Group 1 and 11% of Group 2 (p = 0.0001). There were three procedure-related deaths in Group 1, but none in Group 2.(ABSTRACT TRUNCATED AT 250 WORDS)