The cysteinyl leukotrienes (cys-LTs) are a family of potent lipid mediators of inflammation derived from arachidonic acid. Activation of certain cell types results in the biosynthesis and export of leukotriene (LT) C₄, which then undergoes extracellular metabolism to LTD₄ and LTE₄. LTE₄, the most stable cys-LT, is only a weak agonist for the defined type 1 and type 2 cys-LT receptors (CysLT₁R and CysLT₂R, respectively). We had recognized a greater potency for LTE₄ than LTC₄ or LTD₄ in constricting guinea pig trachea in vitro and comparable activity in eliciting a cutaneous wheal and flare response in humans. Thus, we hypothesized that a vascular permeability response to LTE₄ in mice lacking both the CysLT₁R and CysLT₂R could establish the existence of a separate LTE₄ receptor. We now report that the intradermal injection of LTE₄ into the ear of mice deficient in both CysLT₁R and CysLT₂R elicits a vascular leak that exceeds the response to intradermal injection of LTC₄ or LTD₄, and that this response is inhibited by pretreatment of the mice with pertussis toxin or a Rho kinase inhibitor. LTE₄ is ≈64-fold more potent in the CysLT₁R/CysLT₂R double-deficient mice than in sufficient mice. The administration of a CysLT₁R antagonist augmented the permeability response of the CysLT₁R/CysLT₂R double-deficient mice to LTC₄, LTD₄, and LTE₄. Our findings establish the existence of a third receptor, CysLT_ER, that responds preferentially to LTE₄, the most abundant cys-LT in biologic fluids, and thus reveal a new target for therapeutic intervention.