Measurement of extravascular lung water by a double-indicator dilution technique using cold indocyanine green dye was evaluated in dogs. Pulmonary edema was induced in 11 animals by volume overload; 12 animals served as controls. For each measurement, the 2 indicators (cold dye) were injected into the superior vena cava and detected in the femoral artery. The extravascular thermal volume was calculated from the mean times of the 2 indicator curves. Pretermination measurements of extravascular thermal volume [EVTV] correlated closely with standard gravimetric analysis of pulmonary extravascular tissue weight [PEW] (EVTV = 1.15 PEW + 2.1 ml/kg, n = 21, r = 0.97, P < 0.001). Throughout the experiment, the arterial O2 tension and alveolar-arterial O2 tension gradient correlated poorly with EVTV (linear correlation: r = 0.47, 0.45, respectively). The intrapulmonary shunt correlated better with EVTV (r = 0.72). Nonlinear correlation of EVTV with intravascular pressures (left ventricular filling pressures, colloid oncotic pressures and the pulmonary artery occlusion pressure-colloid oncotic pressure gradient) were more significant than linear relationships. The critical pressures at which lung water rapidly increased in this model occurred at left ventricular filling pressures of 22-27 mm Hg and at pulmonary artery occlusion pressure-colloid oncotic pressure gradients of 25-30 mm Hg. The thermal dye technique appears to provide an accurate measurement of lung water changes in this high pressure edema model.