This review explores the literature dealing with the relation between the upper and lower airways, including the possible link between chronic sinus disease and asthma, and between chronic sinus disease and cough. Imaging studies, microbiology, epidemiology, animal studies, and effects of treatment are discussed. Available studies do not prove that upper airway disease directly causes lower airway pathology. Allergic rhinitis causing nasal blockage needs treatment, as does symptomatic sinus disease. Where there is concurrent disease of the upper and lower airways, both conditions need to be treated adequately. Further research is required to establish the relation between upper and lower airways, and animal models may help to unravel the mechanisms and impact of treatment. Randomized, blinded, controlled trials are needed in both children and adults to assess therapies of chronic sinusitis and the response of asthma.