Rationale: Disruption of endothelial barrier function and neutrophil-mediated injury are two major mechanisms underlying the pathophysiology of sepsis-induced acute lung injury (ALI). Recently we reported that endotoxin induced activation of RhoA in mice lungs that led to the disruption of endothelial barrier and lung edema formation; however, the molecular mechanism of this phenomenon remained unknown. Objective: We reasoned that LIMK1, which participates in the regulation of endothelial cell contractility and is activated by RhoA/Rho kinase pathway, could mediate RhoA-dependent disruption of endothelial barrier function in mouse lungs during ALI. And if that is the case, then attenuation of endothelial cell contractility by downregulating LIMK1 may lead to the enhancement of endothelial barrier function, which could protect mice from endotoxin-induced ALI. Methods and Results: Here we report that LIMK1 deficiency in mice significantly reduced mortality induced by endotoxin. Data showed that lung edema formation, lung microvascular permeability, and neutrophil infiltration into the lungs were suppressed in limk1 mice. Conclusions: We identified that improvement of endothelial barrier function along with impaired neutrophil chemotaxis were the underlying mechanisms that reduced severity of ALI in limk1 mice, pointing to a new therapeutic target for diseases associated with acute inflammation of the lungs. © 2009 American Heart Association, Inc.