The feasibility of using A-mode ultrasound to measure lung density changes was tested in two studies. (A) Wet sponges were used as experimental analogs of the lung. The sponges were weighed. A-mode recordings were obtained by placing the transducer face against a sheet of film in contact with each sponge. Ultrasound penetration increased directly with increasing water content. The range of densities studied corresponds with that which occurs in normal and edematous human lungs. In a second study (B), the depth of ultrasound penetration into 3 postmortem human lungs was measured at increasing levels of inflation by positive intrabronchial pressure. Ultrasound penetration decreased approximately logarithmically with increasing inflation pressure. These relationships indicate a potential application of ultrasound to pulmonary densitometry. This could provide a noninvasive means for clinical estimation of changes in lung water, without the use of ionizing radiation.