Asthma is characterized by eosinophilic and mononuclear cell infiltration, mucous metaplasia, airway remodeling, reversible airflow obstruction, and airway hyperresponsiveness. COPD is typified by nonreversible or incompletely reversible airway obstruction, often accompanied by mucous metaplasia and alveolar destruction. There is considerable overlap in pathogenesis and clinical features between the conditions. However, asthma and COPD may be distinguished by their respective cytokine profiles. Studies in transgenic mice have illuminated the roles of the T helper (Th) 1-mediated cytokine interferon-γ in COPD, supporting the British hypothesis, and the Th2-mediated cytokine interleukin-13 in asthma, supporting the Dutch hypothesis. COPD and asthma may represent disease states along a continuum, with varying degrees of each disease often present in the same patient.