Variability in technique and reporting time may limit radiographic quantitation of extravascular lung water by portable chest x-ray in critically ill patients. Using double indicator dilution technique and a commercially available lung water computer, we measured extravascular thermal volume (ETVL) in 14 patients with x-ray evidence of pulmonary edema and compared these results to x-ray quantitation of pulmonary edema. The diagnosis of pulmonary edema by interpretation of initial x-rays in each patient's series was 64% accurate as 5 of 14 patients had normal ETVL. Estimation of the magnitude of change in ETVL by radiologic interpretation was 42% accurate. No correlation was found between venous admixture Qsp/Qt, pulmonary capillary wedge pressure (WP) or ETVL values. Measurement of ETVL may aid in the care of critically ill patients with suspected pulmonary edema.