Systemic sepsis and pneumonia are common predisposing factors for ARDS, which can serve as the initial manifestation of the multisystem organ failure syndrome. Primary pneumonia that necessitates ICU admission leads to ARDS in approximately 10% of patients. Systemic infection can also lead to ARDS, but when bacteremia alone is present, the risk is low (probably less than 5%). If the septic syndrome with a hemodynamic and end-organ response develops, the ARDS may follow in as many as 40% of patients. When multiple risk factors for acute lung injury are present, the risk of developing ARDS rises dramatically. The septic syndrome, acute lung injury, and multiorgan failure are closely tied to one another because bacterial cell walls can activate inflammatory mediators, such as interleukin-1 and tumor necrosis factor, which can in turn lead to the septic syndrome and inflammatory injury to the lung. Clinical features, more than serum markers, have been the best predictors of whether lung injury will follow sepsis, indicating that the mere presence of mediators alone cannot cause ARDS and that there are individual susceptibility factors in the effects of these mediators. With the advent of monoclonal antibodies and new anti-inflammatory drugs, prevention of progression from sepsis to multiorgan failure may become possible. Pneumonia is the most common infection that complicates ARDS once it is established, and the mortality rate may approach 90%. The existence of acute lung injury, its predisposing conditions, coexisting illnesses, and the therapeutic interventions used for patients with lung injury all can interfere with lung host defenses and set the stage for bacterial infection of the already-injured lung. This infection appears to add to the propagation of the multiple system organ failure that has already begun. In the future, it may become possible to prevent this infection, which would be a welcome development, because currently, we are stymied in our efforts to diagnose and treat pneumonia in the setting of acute lung injury. Preventive efforts will follow from an understanding of the pathogenesis of pneumonia and in the future may include topical antibiotics, selective digestive decontamination, and prophylactic passive immunotherapy.