||BACKGROUND: Although allergic rhinitis and asthma frequently coexist, the nature of this association is poorly understood. Therefore, we examined whether upper and lower airway patency are associated. METHODS: We investigated 221 6-year-old children from the Copenhagen Prospective Study on Asthma in Childhood birth cohort, assessing upper airway patency by acoustic rhinometry before and after alpha-agonist, and lower airway patency by spirometry before and after beta2-agonist. Furthermore, we measured blood eosinophil count, nasal eosinophilia, total IgE, and fraction of exhaled nitric oxide. Associations were investigated by generalized linear models. RESULTS: Decongested nasal airway patency and post-beta2 FEV(1) were significantly associated (P = .007). The association remained significant after adjustments for sex, body size, FVC, and atopic diseases (beta-coefficient 2.85 cm(3); 95% CI, 0.42 to 5.29; P = .02). Baseline values of upper and lower airway patency were also significantly associated (beta-coefficient 0.89 cm(3); 95% CI, 0.26-1.51; P = .01). In addition, blood eosinophil count and nasal eosinophilia were inversely associated with decongested nasal airway patency, beta-coefficient -0.42 cm(3) (95% CI, -0.77 to -0.07; P = .02) and beta-coefficient -0.47 cm(3) (95% CI, -0.89 to -0.05; P = .03), respectively. CONCLUSIONS: We found a strong and consistent association between upper and lower airway patency. This may be due to a common pathology, as suggested by the inverse association between decongested nasal airway patency, blood eosinophil count, and nasal eosinophilia. Alternatively, the association between upper and lower airway patency might reflect a physiologic background for the common comorbidity.