Interest in the pathogenetic mechanisms of acute lung injury has focusd in recent years on the cellular and biochemical mediators of the acute respiratory distress syndrome (ARDS), resulting in an exponential increase in the number of published studies on humoral or cellular factors as potential biological markers of acute lung injury. There are three compelling reasons to search for sensitiveand specific biological markers of ARDS. First, these markers may improve the prediction of ARDS in high-risk clinical conditions such as sepsis, aspiration of gastric contents, pneumonia, and severetrauma. Second, they may provide new insights into the pathogenesis of ARDS. Finally, they may help to predict the outcome of patients once ARDS has developed. The primary purpose of this review is to summarize and critique the value of biological markers measured in plasma or in