The thermal-dye technique for the measurement of ELW is available for clinical and experimental use. This method is safe and can be performed serially in an individual patient. Although it is invasive, it requires only a central venous catheter and an arterial catheter, which are often already in place for routine hemodynamic monitoring and management. The procedure is accurate under a variety of conditions. Two obstacles argue against its routine application. First, the reliability of this approach appears to be seriously compromised when there are areas of edematous lung with poor blood perfusion. This includes aspiration and perhaps other forms of acute lung injury. Second, it remains to be demonstrated how useful routine measurements of ETVL, even if accurate, are in clinical management. However, regardless of the accuracy of the various methods for determination of ELW, the process of their development has significantly enhanced our knowledge of pulmonary edema formation and fluid distribution. We are at a transition point of soon being able to determine ELW easily. Techniques of reducing ELW can now be assessed directly in patients with pulmonary edema. As these therapies are developed and understood, the demand for the measurement of ELW will become a greater part of clinical medicine, and may stimulate further refinement of methods for quantitating lung water.