The acute respiratory distress syndrome (ARDS) is an inflammatory disease process of the lungs as a response to both direct and indirect insults, characterized clinically by severe hypoxemia, reduced lung compliance, and bilateral radiographic infiltrates. ARDS is caused by an insult to the alveolar-capillary membrane that results in increased permeability and subsequent interstitial and alveolar edema. The mechanisms by which a wide variety of insults can lead to this syndrome are not clear. It is useful to think of the pathogenesis of ARDS as a result of two different pathways: a direct insult on lung cells and an indirect insult as a result of an acute systemic inflammatory response.