Using an easily performed and reproducible test, patients with tight mitral stenosis showed a statistically significant decrease in vital capacity immediately after two step exercise. A similar decrease was also found in patients with left ventricular failure. These findings correlated well with increased pulmonary capillary wedge pressure and/or left ventricular diastolic pressure induced by leg exercise. The availability and simplicity of the immediate post exercise vital capacity recommend it as a practical clinical test to assess disease affecting the left side of the heart.