Background: Spirometry for early detection of chronic obstructive pulmonary disease (COPD) and smoking cessation is criticised because of the potential negative effects of labelling with disease. Aim: To assess the effects of opinions of smokers with mild to moderate COPD on the effectiveness of spirometry for smoking cessation, the justification of early detection of airflow limitation in smokers and the impact of confrontation with COPD. Design: Qualitative study with data from a randomised controlled trial. Setting: General population of Dutch and Belgian Limburg. Methods: Semistructured ethical exit interviews were conducted with 205 smokers who were motivated to quit smoking and had no prior diagnosis of COPD but were detected with airflows limitation by means of spirometry. They received either (1) counselling, including labelling with COPD, plus with nortriptyline for smoking cessation, (2) counselling excluding labelling with COPD, plus nortriptyline for smoking cessation or (3) care as usual for smoking cessation by the general practitioner, without labelling with COPD. Results: Of the participants, 177 (86%) agreed or completely agreed that it is justified to measure lung function in heavy smokers. These participants argued that measuring lung function raises consciousness of the negative effects of smoking, helps to prevent disease or increases motivation to stop smoking. Most of the 18 participants who disagreed argued that routinely measuring lung function in smokers would interfere with freedom of choice. Conclusions: Labelling with disease is probably a less important issue in the discussion about the pros and cons of early detection of COPD.