Spirometry is used worldwide to diagnose respiratory disease, and it is a validated measure to assess airway obstruction. Irreversible airway obstruction is the defining feature of chronic obstructive pulmonary disease (COPD). Thus, an FEV(1)/FVC ratio <70% is used to diagnose COPD, and the severity is thereafter based on the level of FEV(1). This definition is widely used in clinical practice and research, yet may lead to confusion with respect to the diagnosis associated with the presence of airway obstruction. The three main reasons are the following: (1) fixed airflow obstruction may be the result of specific diagnoses such as cystic fibrosis; (2) FEV(1)/FVC ratio changes with ageing, and it is therefore inappropriate to use the same ratio at 40 and 90 years, leaving aside gender differences; (3) even when specific diagnoses are excluded, fixed airflow obstruction may be the end-stage of many different underlying processes. The authors believe that they have strong arguments that a COPD diagnosis based solely on spirometric values is nonsense. More sophisticated lung function tests, such as plethysmography, forced oscillation and lung clearance index, may help further to delineate the characteristics of low lung function. However, these are not feasible in most clinical contexts and in epidemiologic studies. Therefore, the authors throw down the gauntlet: spirometry is an essential tool in patient evaluation but dangerous for disease diagnosis, and the term COPD should only be used in the appropriate clinical (diagnostic) context.