In studies conducted since the epidemic of allergic disorders began, at least 40% of young patients with allergic rhinitis have been found to have concurrent asthma, and up to 100% of young patients with allergic asthma have been found to have concurrent allergic rhinitis. Many patients with occupational or nonsteroidal anti-inflammatory drug-induced asthma also have concurrent allergic rhinitis. In addition to being associated epidemiologically (occurring in patients with similar genetic background and triggered by similar provoking factors), allergic rhinitis and asthma are associated anatomically, physiologically, immunopathologically, and by their response to therapeutic interventions. Anatomically, both the upper and the lower airways are lined with ciliated columnar epithelium containing mucus-secreting goblet cells. Physiologically, they are connected not only by the nasobronchial reflex, but also by the adverse effects on the lower airways produced when nasal congestion results in mouth breathing and loss of nasal air-conditioning (warming, humidification, and filtration of inspired air). The underlying immunopathological process is similar in allergic rhinitis and asthma. It involves not only the immediate hypersensitivity (Type I) allergic response, but also persistent allergic inflammation (the Type IVa2 response in the revised Gell and Coombs classification). In addition, the systemic immunologic response to intranasal or orally-inhaled allergens is similar in allergic rhinitis and asthma. Down-regulation of allergic inflammation by allergen avoidance, allergen-specific immunotherapy, and medications such as H1-antihistamines, leukotriene modifiers, intranasal/inhaled glucocorticoids or novel immunomodulators such as anti-IgE is the key to managing both disorders. ‘Combined allergic rhinitis/asthma syndrome’, ‘allergic rhinobronchitis’, ‘the united airways’, ‘one airway, one disease’, and other phrases in current usage lead to increased awareness that persistent allergic inflammation occurs throughout the upper and lower airways of patients with concurrent allergic rhinitis and asthma. Terms involving the words ‘allergic’ or ‘asthma’ are preferable to nonspecific terms such as ‘united airways’ or ‘one airway, one disease’. Additional studies of the immunopathophysiological processes that link the upper and lower airways are needed.