The pathophysiological role of mechanical and myocardial factors for impairment of left ventricular performance in mitral stenosis is still not clear. To investigate this controversy, 27 patients of mitral stenosis with left ventricular ejection fraction < 50% were studied. Patients were divided into two groups: Group 1: 20 patients, left ventricular ejection fraction improved to > 50% after valvuloplasty, and Group 2: 7 patients, left ventricular ejection fraction still < 50% after valvuloplasty. The clinical and hemodynamic characteristics were comparable for the two groups before valvuloplasty. Follow-up catheterization done one week later showed similar changes in mitral valve area, cardiac index, pulmonary pressure, left ventricular end-diastolic volume index and systemic vascular resistance between the two groups. However, left ventricular end-systolic volume was significantly decreased after valvuloplasty in Group 1 but not in Group 2, resulting in significantly higher ejection fraction in Group 1 than in Group 2. Postoperatively, regional wall motion scores were lower in Group 1 than in Group 2 (2.0 +/- 0.6 vs. 2.7 +/- 0.5 at the anterolateral wall, P = 0.002; 1.9 +/- 0.6 vs. 2.9 +/- 0.4 at the posterobasal wall, P = 0.0003). Most of our mitral stenosis patients with impaired left ventricular ejection fraction showed improvement after mitral valvuloplasty had released the mechanical obstruction. However, in some patients, impaired ejection fraction persisted after valvuloplasty, suggesting the mechanism of myocardial failure. Thus, both myocardial and mechanical factors play important roles in the pathogenesis of left ventricular ejection performance impairment.