CD74 is expressed on the cell surface of pulmonary macrophages and contributes to macrophage migration inhibitory factor (MIF)-induced inflammatory response in acute lung injury (ALI). A circulating form of CD74 (soluble CD74, sCD74) was recently discovered in autoimmune liver disease. Using two murine ALI models and cells culture, we examined the presence of sCD74 in circulation and alveolar space and preliminarily assessed the biological function of sCD74. The concentrations of sCD74 were increased in serum and bronchoalveolar lavage fluids (BALF) of murine ALI models. The elevated levels of sCD74 in BALF positively correlated with lung permeability and inflammation. In addition, sCD74 is secreted by macrophages in response to MIF stimulation and itself can stimulate the production of inflammatory cytokines. Our clinical study confirmed some findings of basic research. Moreover, we also found Day 3 serum sCD74 levels were associated with worse clinical outcomes. In conclusion, higher serum sCD74 levels may reflect more severe lung injury and may be used to help physicians determine prognosis of acute respiratory distress syndrome (ARDS).