Disturbances of the upper and lower airways frequently coexist, and the association between allergic rhinitis and asthma is an example of that. The relationship between allergic rhinitis and asthma probably occurs because both, nasal and bronchial mucosas are elements of a "united airway", and on the other hand, allergic rhinitis and asthma are manifestations of a common allergic disease. Allergic rhinitis and asthma are not only statistically associated, but have pathophysiological and clinical similarities. Allergic rhinitis is itself a risk factor for the development of asthma, but additionally may confound the diagnosis of asthma and may exacerbate coexisting asthma. The management of allergic rhinitis, mainly with the use of intranasal corticosteroids, improve asthma symptoms and lung function in asthmatic patients. Several mechanisms have been proposed to link the nose and bronchi, which include: postnasal drip of inflammatory cells and pro-inflammatory molecules; a possible nasobronchial neural reflex; an increased exposure of the lower airways to dry and cold air as well as aeroallergens because the mouth breathing secondary to nasal obstruction; and an increased susceptibility to rhinovirus infection secondary to an increased ICAM-1 expression in the nasal mucosa of patients with allergic rhinitis. A better understanding of the rhinitis-asthma relationship nature might allow the creation of better strategies for the integral treatment of patients with these diseases.