A method for measuring content of water in the lung is described. The method utilizes changes in the amplitude and phase of the microwave signal transmitted through the thorax. The phase of the transmission coefficient is particularly sensitive and provides a means of detecting and monitoring of pulmonary edema. Theoretical predictions of sensitivity were confirmed experimentally in agar models of the thorax. The optimal frequency lies between 740 MHz and 1.5 GHz. A 50-ohm open-transmission line was designed and used to couple 915-MHz energy into tissues with minimal external radiation. Other advantages of this strip-line concept include its low mass, small size, and flat configuration, which render it utile for clinical purposes. The feasibility of the method was also evaluated in experiments on dogs. In a typical dog experiment, the phase of the microwave-transmitted signal changed by an average value of 2° for each 10 milliliters of infused blood. These results provide strong encouragement that incipient pulmonary edema can be detected before clinical emergencies arise, thus providing the physician a safe and sensitive tool for diagnosis of pulmonary function.