The diagnostic labels used to characterise common airway diseases have always been a problem. The term asthma implies the presence of variable airflow obstruction; however, objective demonstration can be difficult. Commonly used tests, such as spirometry or serial peak-flow measurements, are neither sensitive nor specific, especially in patients with mild disease and normal or near-normal lung function, or in those with fixed airflow obstruction. Conversely, the term chronic obstructive pulmonary disease (COPD) implies largely irreversible airflow obstruction, yet clinicians may attempt to confirm the presence of reversibility, and having done so, label the disease as having an “asthmatic” component. Furthermore, the use of specific diagnostic labels implies a probable natural history, and influences expectations about treatment outcomes.