The adult or acute respiratory distress syndrome (ARDS) is an increased permeability pulmonary edema that occurs most often as a complication of bacterial sepsis, aspiration of gastric contents, or massive trauma. Clinically, the syndrome is recognized as acute respiratory failure with bilateral infiltrates on chest radiograph, a marked oxygenation defect, and normal or nearly normal cardiac function. Since its first comprehensive description > 25 yrs ago, the reported mortality rate of ARDS has remained high. Current series describe mortality rates in the range of 50% to 60%. The major cause of death in patients with ARDS seems to be bacterial sepsis and multiple organ dysfunction. Predictors of survival and mortality have been studied. Older patients (> or = 60 yrs) who develop bacterial sepsis and multiple organ dysfunction are at high risk to die. Younger patients who develop ARDS from trauma or other noninfectious causes have a better prognosis. Persistent metabolic acidosis on the first day of ARDS is a particularly poor prognostic sign.