We measured pulmonary and systemic flows in 22 patients with mitral stenosis and in 7 controls. In patients with mitral stenosis, pulmonary flow index averaged 2.33 ± 0.41 l/min/m<sup>2</sup> and systemic flow index averaged 2.15 ± 0.601/min/m<sup>2</sup>, p = 0.045. There was a strong correlation between the difference in pulmonary and systemic flow indexes and the difference in mean left and right atrial pressure (r = 0.749, p = 0.00008). After a successful dilatation in 17 patients, there was a significant drop in the difference between pulmonary and systemic flow indexes (0.26 ± 0.41 vs. 0.07 ± 0.371/min/m<sup>2</sup>, p = 0.048). We conclude that chronic elevation of left atrial pressure leads to a left to right shunt probably through bronchial veins.