A mobile lung densitometer using conventional x‐ray tubes, NaI detectors, and principles of two‐source, two‐detector Compton scattered densitometry, is described. The device is capable of one to two per second density measurements from a 45‐cm3volume with a precision of 5%. The expectedin vivoaccuracy (2%–3%) is determined by using an anthropomorphic phantom with replaceable lung inserts. The unintentional detection of multiple‐scattered x rays results in a small density‐dependent error. This error is predictable and relatively insensitive to differences in surrounding absorbers such as the chest wall. With this device, dynamicin vivodensitometry of the lung in the clinical laboratory and intensive care unit will be possible.