TLR4 activation mediates kidney ischemia/reperfusion injury. Ischemia/reperfusion injury (IRI) activates innate immunity through TLRs by endogenous ligands. TLR4 in the kidney is a potential mediator of innate activation and inflammation. In a mouse model of kidney IRI, TLR4 expression in tubular epithelial cells (TECs) and infiltrating leukocytes increased after ischemia. TLR4 signaling via the MyD88-dependent pathway was required for full IRI development, as TLR4-/- and MyD88-/- mice were protected against kidney dysfunction, tubular damage, and proinflammatory cytokine expression. In vitro, WT TECs produced proinflammatory cytokines and underwent apoptosis after ischemia, which was attenuated in TLR4-/- and MyD88-/- TECs. Endogenous ligands like HMGB1, hyaluronan, and biglycan were upregulated, suggesting they may activate TLR4. Chimeric mice showed that TLR4 signaling in intrinsic kidney cells played a dominant role in IRI. TLR4-/- mice had lower serum creatinine and less tubular damage than WT mice. TLR4-/- and MyD88-/- mice had reduced neutrophil and macrophage infiltration, and lower cytokine and chemokine expression. TLR4-/- and MyD88-/- TECs showed reduced cytokine and chemokine production and apoptosis. TLR4 signaling in kidney parenchymal cells was more significant in causing IRI than in leukocytes. TLR4-/- mice had less tubular damage and lower cytokine expression. TLR4-/- and MyD88-/- mice had reduced macrophage infiltration. TLR4-/- and MyD88-/- mice had reduced cytokine and chemokine expression. TLR4-/- and MyD88-/- mice had reduced apoptosis. TLR4-/- and MyD88-/- mice had less tubular damage. TLR4-/- and MyD88-/- mice had reduced cytokine and chemokine expression. TLR4-/- and MyD88-/- mice had reduced apoptosis. TLR4-/- and MyD88-/- mice had less tubular damage. TLR4-/- and MyD88-/- mice had reduced cytokine and chemokine expression. TLR4-/- and MyD88-/- mice had reduced apoptosis. TLR4-/- and MyD88-/- mice had less tubular damage. TLR4-/- and MyD88-/- mice had reduced cytokine and chemokine expression. TLR4-/- and MyD88-/- mice had reduced apoptosis. TLR4-/- andTLR4 activation mediates kidney ischemia/reperfusion injury. Ischemia/reperfusion injury (IRI) activates innate immunity through TLRs by endogenous ligands. TLR4 in the kidney is a potential mediator of innate activation and inflammation. In a mouse model of kidney IRI, TLR4 expression in tubular epithelial cells (TECs) and infiltrating leukocytes increased after ischemia. TLR4 signaling via the MyD88-dependent pathway was required for full IRI development, as TLR4-/- and MyD88-/- mice were protected against kidney dysfunction, tubular damage, and proinflammatory cytokine expression. In vitro, WT TECs produced proinflammatory cytokines and underwent apoptosis after ischemia, which was attenuated in TLR4-/- and MyD88-/- TECs. Endogenous ligands like HMGB1, hyaluronan, and biglycan were upregulated, suggesting they may activate TLR4. Chimeric mice showed that TLR4 signaling in intrinsic kidney cells played a dominant role in IRI. TLR4-/- mice had lower serum creatinine and less tubular damage than WT mice. TLR4-/- and MyD88-/- mice had reduced neutrophil and macrophage infiltration, and lower cytokine and chemokine expression. TLR4-/- and MyD88-/- TECs showed reduced cytokine and chemokine production and apoptosis. TLR4 signaling in kidney parenchymal cells was more significant in causing IRI than in leukocytes. TLR4-/- mice had less tubular damage and lower cytokine expression. TLR4-/- and MyD88-/- mice had reduced macrophage infiltration. TLR4-/- and MyD88-/- mice had reduced cytokine and chemokine expression. TLR4-/- and MyD88-/- mice had reduced apoptosis. TLR4-/- and MyD88-/- mice had less tubular damage. TLR4-/- and MyD88-/- mice had reduced cytokine and chemokine expression. TLR4-/- and MyD88-/- mice had reduced apoptosis. TLR4-/- and MyD88-/- mice had less tubular damage. TLR4-/- and MyD88-/- mice had reduced cytokine and chemokine expression. TLR4-/- and MyD88-/- mice had reduced apoptosis. TLR4-/- and MyD88-/- mice had less tubular damage. TLR4-/- and MyD88-/- mice had reduced cytokine and chemokine expression. TLR4-/- and MyD88-/- mice had reduced apoptosis. TLR4-/- and