The link between upper and lower respiratory tracts has been repeatedly observed in the past 50 years but only carefully investigated during the past decade. Several clinical and experimental observations suggested the hypothesis of the unity of upper and lower airways (allergic rhinobronchitis or united airways disease). The relationships between rhinitis (and sinusitis) and asthma also include non-epidemiological aspects such as viral infections and bronchial hyperreactivity. The hypotheses have been confirmed by means of epidemiological observations, functional and immunological evidence and, indirectly, by observing the effects of drugs used mainly for rhinitis on asthma symptoms. In this article, therefore, we collected and reviewed the most relevant experimental results available to support the hypothesis for united airways disease and the studies conducted on the possible mechanisms of nose-lung interaction.