The prevalence of asthma and allergic rhinitis is increasing in the general population, and a high proportion of new patients have coexisting upper and lower airway disease. Estimates show that 60 to 78% of patients who have asthma have coexisting allergic rhinitis. During the past decade, our understanding of asthma and allergic rhinitis has evolved. The historic perspective of these allergen-induced disorders as distinct and separate entities is being displaced by current thinking that they are described better as a continuum of inflammation involving one common airway. Therefore, traditional therapies originally indicated for allergic rhinitis and asthma are being reassessed to explore their potential utility in both upper and lower airway diseases. Recently, there has been a renewed interest in the role that histamines play in lower airway disease, and interest is increasing in the theory that leukotrienes, which are far more potent inflammatory mediators than histamines, play a role in upper airway disease. Given the pivotal role that leukotrienes play as potent inflammatory mediators in the pathophysiologic state of inflammation of both airways, leukotriene receptor antagonists recently have emerged as important therapeutic advances that have potential clinical utility in both asthma and allergic rhinitis.