This is a review of some of the main findings obtained by positron emission tomography (PET) concerning the pathophysiology of acute respiratory distress syndrome (ARDS) and acute lung injury. PET (which is nowadays often combined with computed tomography) is a functional imaging technique based on the detection of a labeled molecule administered to a subject. Based on the molecule used, different lung functions can be imaged. Examples include inhaled, labeled nitrogen, which allows us to visualize regional aeration and ventilation, whereas lung perfusion has been studied by means of labeled water or by injected nitrogen dissolved in saline. With this latter technique, a global assessment of regional gas exchange is possible. Administration of [18F]FDG facilitates the imaging of cellular metabolic activity, reflecting an acute neutrophil-sustained inflammatory process. This technique has been used in experimental ARDS and, more recently, in patients. It showed, for example, that inflammatory activity of the lungs is markedly increased even in "normally aerated" regions at levels that are, in some cases, even higher than in the non-aerated regions.