Radionuclide lung perfusion imaging was performed on 27 patients with valvular disease of the left heart. The ratio of upper to total counts for the lungs, determined by computer, was correlated against pulmonary vascular mean pressures. A close correlation (r = 0.91) was obtained against pulmonary wedge pressure. After corrective cardiac surgery upper/total ratios fell towards normal in four patients in whom pulmonary vascular pressures were measured and the correlation persisted. This simple non-invasive index can be used to follow changes in pulmonary venous hypertension.