The link between upper and lower airways has been observed in the past, but only carefully investigated during the last years. Allergic rhinitis and asthma are often comorbid conditions. Its relationship is supported by epidemiological, anatomical and physiological, immunopathological, clinical and therapeutic studies, mostly related to allergic rhinitis. Rhinitis and asthma occur together at rates that greatly exceed what would be expected from the baseline prevalence of each disorder alone. Many researchers have considered rhinitis as a risk factor for developing asthma. Also, the severity of rhinitis has been directly correlated with the severity of asthma although less evidence supporting the reverse impact. Different clinical and experimental observations suggested the hypothesis of the unity of upper and lower respiratory tract. Evidence suggests that rhinitis and asthma are different facets of a broader systemic inflammatory process involving upper and lower airways. Treatment of rhinitis simultaneously produces a favorable effect on symptoms of asthma and concurrent improvement in lung function and bronchial hyperresponsiveness. In this article, we review the most relevant epidemiological, genetic and clinical evidence linking rhinitis and asthma, the possible mechanisms of the nose-lung interaction and the rational basis of a set treatment that support the hypothesis of "one airway, one disease "considering aspects not yet clarified on the subject.