To understand the significance of measured extravascular thermal volume of the lung (ETVL) in chronic pulmonary diseases, we measured ETVL using the double-indicator dilution technique in 17 emphysematous patients and 15 patients with pulmonary fibrosis. The level of ETVL in emphysema is significantly correlated with parameters reflecting the severity of the disease, including FEV1.0% (r = 0.54, p < 0.05), DLCO/VA (r = 0.54, p < 0.05), PaO2 (r = 0.61, p < 0.01), and mean pulmonary artery pressure (r = -0.61, p < 0.01). In pulmonary fibrosis, a significant inverse correlation was found between ETVL and PaO2 (r = -0.64, p < 0.02), DLCO/VA (r = -0.58, p < 0.05), and %VC (r = -0.61 p < 0.02). The mean level of ETVL in both groups of patients was lower than previously reported normal values. These results suggest that ETVL may reflect the decrease in pulmonary vasculature in emphysematous patients, while it may signify a decrease of pulmonary vasculature and increased volume of pulmonary interstitial tissue surrounding the pulmonary vasculature in pulmonary fibrosis. We conclude that ETVL may represent a good estimate of the condition of the pulmonary vasculature and interstitial tissue volume in patients with chronic pulmonary diseases.