Eicosanoids derived from arachidonic acid are lipid mediators that are involved in diverse inflammatory and immune responses. The cysteinyl leukotrienes, leukotriene (LT) C4, LTD4, and LTE4, are a group of bioactive eicosanoids that are particularly implicated in pathobiology of bronchial asthma. LTC4 synthase, a membrane protein, is the critical enzyme for cysteinyl leukotriene biosynthesis. There are two distinct G protein-coupled receptors for the cysteinyl leukotrienes, termed CysLT1 receptor and CysLT2 receptor. Recent studies suggest that the two receptors are non-redundant in functions and that additional cysteinyl leukotriene receptors exist.