The exercise stress test with a bicycle ergometer was performed for 31 patients with mitral stenosis and for 10 normal subjects. The patients were categorized in two groups. Group 1 consisted of 16 patients having indication for surgical intervention and Group 2 consisted of 15 patients without such surgical indications. Oxygen uptakes at the anaerobic threshold and at peak exercise (MAX) were assessed by percent attainment of the predicted normal value from Posner's equation. Heart rates during exercise did not differ between the two groups. However, Group 1 had significantly smaller values of percent attainment of oxygen uptake both at the anerobic threshold and peak exercise than the controls and Group 2. Oxygen pulses in Group 1 were also significantly less than in the controls or Group 2. Seven cases were reassessed six months or more after surgery including open mitral commissurotomy in two and mitral valve replacements in five. The improvement of oxygen pulse showed a statistical significance. Percent oxygen uptake attainment was also significantly improved both at the anaerobic threshold and at peak exercise. Percent attainment of oxygen uptake in mitral stenosis differed significantly according to the NYHA class both at the anaerobic threshold and peak exercise. These values are considered useful for making decisions for surgical treatment in borderline cases.