The link between upper airway disease (allergic rhinitis and sinusitis) and lower airway disease (asthma) has long been of interest to physicians. Many epidemiological and pharmacological studies have provided a better understanding of pathophysiologic interrelationship between allergic rhinitis and asthma. The vast majority of patients with asthma have allergic rhinitis, and rhinitis is a major independent risk factor for asthma in cross-sectional and longitudinal studies. The association between sinusitis and asthma has long been appreciated. Through the recent evidences, allergic rhinitis, sinusitis, and asthma may not be considered as different diseases but rather as the expression in different parts of the respiratory tract of same pathological process in nature. Various mechanisms have been proposed to explain the relationship between asthma and upper airway diseases, but the underlying mechanisms are not completely discovered. The implications for the one-airway hypothesis are important not only academically but also clinically for diagnostic and therapeutic purposes.