The pathogenesis of chronic obstructive pulmonary disease (COPD) is characterized by pulmonary inflammation associated with lung neutrophilia and elevated levels of pro-inflammatory mediators in the bronchoalveolar lavage fluid or sputum of patients. Recent findings revealed that mitogen-activated protein kinase (MAPK) signaling cascade is involved in the inflammatory response of lung injury. In the present study we could elucidate the role of extracellular signal-related MAPK in the murine model of LPS-induced acute lung injury by using U0126, a specific inhibitor of MEK1/2, upstream kinases of ERK. Phosphorylation of ERK was inhibited by U0126 in vivo as well as in vitro. In freshly isolated human peripheral blood mononuclear cells U0126 dose-dependently blocked the release of IL-2 and TNF-alpha. For in vivo studies mice were exposed to aerosolized LPS to induce an acute lung injury mimicking some aspects of COPD. This led to a recruitment of neutrophils to the lung and to the release of pro-inflammatory cytokines into bronchoalveolar lavage. Pretreatment of mice with U0126 significantly reduced lung neutrophilia and diminished levels of TNF-alpha and chemotactic MIP-2 and KC in bronchoalveolar fluid. U0126 also decreased albumin levels in BAL fluid, a marker of vascular leakage. Histological examination of lung tissues revealed that ERK MAPK inhibition using U0126 efficiently attenuated LPS-induced pulmonary inflammatory responses. These data suggest that ERK signaling plays an important role in acute lung injury and pharmacologic inhibition of ERK provides a promising new therapeutic strategy for lung inflammatory diseases and in particular COPD.