Dysfunction of the upper and lower airways frequently coexist, and they appear to share key elements of pathogenesis. Data from epidemiologic studies indicate that nasal symptoms are experienced by as many as 78% of patients with asthma and that asthma is experienced by as many as 38% of patients with allergic rhinitis. Studies also have identified a temporal relation between the onset of rhinitis and asthma, with rhinitis frequently preceding the development of asthma. Patients with allergic rhinitis and no clinical evidence of asthma commonly exhibit nonspecific bronchial hyperresponsiveness. The observation that management of allergic rhinitis also relieves symptoms of asthma has heightened interest in the link between these diseases. Intranasal corticosteroids can prevent increases in nonspecific bronchial reactivity and asthma symptoms associated with seasonal pollen exposure. Similarly, among patients with perennial rhinitis, daily asthma symptoms, exercise-induced bronchospasm, and bronchial responsiveness to methacholine are reduced after administration of intranasal corticosteroids. Antihistamines, with or without decongestants, reduce seasonal rhinitis symptoms, asthma symptoms, and objective measurements of pulmonary function among patients with rhinitis and asthma. The mechanisms that connect upper and lower airway dysfunction are under investigation. They include a nasal-bronchial reflex, mouth breathing caused by nasal obstruction, and pulmonary aspiration of nasal contents. Nasal allergen challenge results in increases in lower airway reactivity within 30 minutes, suggesting a neural reflex. Improvements in asthma associated with increased nasal breathing may be the result of superior humidification, warming of inspired air, and decreased inhalation of airborne allergens. Postnasal drainage of inflammatory cells during sleep also may affect lower airway responsiveness. A link between allergic rhinitis and asthma is evident from epidemiologic, pathophysiologic, and clinical studies. Future research, however, is needed to determine whether nasal therapy can alter the natural history of asthma.