Using radioactive oxygen (O15), carbon monoxide and carbon dioxide with external counting over the chest, it is possible to measure perfusion and diffusion per unit of lung volume. Patients with mitral stenosis, particularly those with high pulmonary artery pressures, may show a higher blood flow through the upper zone of the lung than the lower. This distribution of blood flow is the reverse of that found in normal subjects. The observed changes in blood flow are consistent with the radiological and pathological changes in the pulmonary vascular tree in mitral stenosis.