Systemically bioavailable leukotriene receptor antagonists (LTRAs) can reduce the essential components of allergic inflammation in allergic rhinitis (AR) and asthma by blocking cysteinyl leukotriene (CysLT) activity, resulting in a wide range of clinical effects. CysLTs, mediators, and modulators in the pathophysiology of asthma and AR are a key target for therapy because they modulate production of hemopoietic progenitor cells, survival and recruitment of eosinophils to inflamed tissue, activity of cytokines and chemokines, quantity of exhaled NO, smooth-muscle contraction, and proliferation of fibroblasts. The mechanism of action of LTRAs leads to their effects on systemic allergic inflammatory processes.