The sensitivity of basal hemodynamic data and the value of those observed during exercise in the diagnosis of severe mitral stenosis (MS) were investigated by comparing the results under basal conditions and during dynamic exercise with the operative findings in 55 patients with pure MS, aged between 18 and 73 years (average, 43 years), 29 with mild stenosis (Group I) and 26 with severe stenosis (Group II). Under basal conditions, mean pulmonary capillary pressure (PCP) was significantly higher in Group II than in Group I (15.7 +/- 4.4 mm Hg compared to 12.2 +/- 3.5 mm Hg, p less than 0.01). The same difference was observed in mean pulmonary arterial pressure: Group I, 18.2 +/- 5.4 mm Hg; Group II: 23.0 +/- 6.1 mm Hg, p less than 0.01; but cardiac and systolic indices were comparable in the two groups. On exercise, only PCP (Group I: 29.6 +/- 6.8; Group II: 34.7 +/- 4.9 mm Hg) and systolic index (Group I: 39.9 +/- 10.0; Group II: 33.4 +/- 8.2 ml/m2) were significantly different (p less than 0.01). Pulmonary capillary and arterial pressures rose in the same proportion in the two groups on exercise but systolic index fell in severe MS (-4 p. 100) and rose in mild MS (+30 p. 100) (p less than 0.001). The formulae for calculating mitral valve surface area only give discriminating results between the two groups when the hemodynamic data on exercise are used.(ABSTRACT TRUNCATED AT 250 WORDS)