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The Th2 cell-dependent inflammatory response is a central component of asthma, and the ways in which it is regulated is a critical question. The cysteinyl leukotrienes (cys-LTs) are 5-lipoxygenase pathway products implicated in asthma, in particular, by their function as smooth muscle constrictors of airways and microvasculature. To elucidate additional roles for cys-LTs in the pathobiology of pulmonary inflammation, we used an OVA sensitization and challenge protocol with mice lacking leukotriene C4 synthase (LTC4S), the terminal enzyme for cys-LT generation. Ag-induced pulmonary inflammation, characterized by eosinophil infiltration, goblet cell hyperplasia with mucus hypersecretion, and accumulation and activation of intraepithelial mast cells was markedly reduced in LTC4Snull mice. Furthermore, Ag-specific IgE and IgG1 in serum, Th2 cell cytokine mRNA expression in the lung, and airway hyperresponsiveness to methacholine were significantly reduced in LTC4Snull mice compared with wild-type controls. Finally, the number of parabronchial lymph node cells from sensitized LTC4Snull mice and their capacity to generate Th2 cell cytokines ex vivo after restimulation with Ag were also significantly reduced. In contrast, delayed-type cutaneous hypersensitivity, a prototypic Th1 cell-dependent response, was intact in LTC4Snull mice. These findings provide direct evidence of a role for cys-LTs in regulating the initiation and/or amplification of Th2 cell-dependent pulmonary inflammation.