Toll-like receptors (TLRs) of the innate immune system contribute to non-infectious inflammatory processes. We employed a murine model of hilar clamping (one hour) with reperfusion times between 15 minutes and 3 hours in TLR4-sufficient (C3H/OuJ) and TLR4-deficient (C3H/HeJ) anesthetized mice with additional studies in chimeric, MyD88-, and TLR4-deficient mice to determine the role of TLR4 in lung ischemia-reperfusion injury. Human pulmonary microvascular endothelial monolayers were subjected to simulated warm ischemia and reperfusion with and without CRX-526, a competitive TLR4 inhibitor. Functional TLR4 solely on pulmonary parenchymal cells, not bone marrow-derived cells, mediates early lung edema following ischemia-reperfusion independent of MyD88. Activation of MAP kinases and NF-B was significantly blunted and/or delayed in lungs of TLR4-deficient mice as a consequence of ischemia-reperfusion injury, but edema development appeared to be independent of activation of these signaling pathways. Pre-treatment with a competitive TLR4 inhibitor prevented edema in vivo and reduced actin cytoskeletal rearrangement and gap formation in pulmonary microvascular endothelial monolayers subjected to simulated warm ischemia and reperfusion. In addition to its well-accepted role to alter gene transcription, functioning TLR4 on pulmonary parenchymal cells plays a key role in very early and profound pulmonary edema in murine lung ischemia-reperfusion injury. This may be due to a novel mechanism - regulation of endothelial cell cytoskeleton affecting microvascular endothelial cell permeability.