Technical Abstract: Although inflammation plays a central role in the pathogenesis of acute lung injury (ALI), the molecular mechanisms underlying inflammatory responses in ALI are poorly understood, and therapeutic options remain limited. The CCAAT/enhancer-binding protein (C/EBP) gamma and -gamma have been implicated in the regulation of inflammatory mediators, while the role of C/EBP in inflammation remains elusive. C/EBP beta, -delta, and -gamma are all expressed in the lung; however, their function in lung are not clear. Here, using two models of acute lung injury, we demonstrate that C/EBP beta, -delta, and -gamma play important regulatory roles in lung inflammatory responses. We show that C/EBP gamma deficiency significantly reduced IgG immune complex-stimulated inflammatory responses in alveolar macrophages and injured lung, whereas C/EBP gamma is critical for LPS-induced acute lung injury. Moreover, overexpression of C/EBP gamma in lung using a recombinant adenovirus encoding murine C/EBP gamma resulted in substantial decreases of lung inflammatory injury induced by both IgG immune complex and LPS. These findings identify the same family of C/EBP transcription factors as critical mediators of both the pro- and anti-inflammatory mechanisms in acute lung injury.