Leukotrienes (LTs), including cysteinyl LTs (CysLTs) and LTB(4), are potent lipid mediators that have an important pathophysiological role in asthma and allergic rhinitis. Most of the effects of CysLTs that are relevant to the pathophysiology of asthma are mediated by the activation of the CysLT(1) receptor, one of the receptor subtypes for CysLTs. LTB(4) might be functionally involved in the development of airway hyperresponsiveness, acute and severe asthma and allergic rhinitis. CysLT(1) receptor antagonists can be given as monotherapy or in addition to inhaled glucocorticoids. The potential anti-remodeling effect of CysLT(1) receptor antagonists might be relevant for preventing or reversing airway structural changes in asthmatic patients. Here, we examine the role of LTs in asthma and allergic rhinitis, and the therapeutic implications of the pharmacological modulation of the LT pathway for allergic airway disease.