The aim was to compare the diagnosis of COPD among smokers according to different international guidelines and to compare the outcome when using slow (SVC) and forced vital capacity (FVC). In order to find current smokers a questionnaire was sent to persons who had been on sick leave for more than two weeks. Those who smoked more than 8 cigarettes per day were invited to perform a spirometry. Totally 3,887 spirometries were performed. In this sample 10.2% fulfilled the NICE COPD-criteria, 14.0% the GOLD COPD-criteria and 21.7% the ERS COPD criteria. The diagnosis according to NICE and GOLD guidelines is based on FVC and in the ERS guidelines the best value of either SVC or FVC is used. Thus, substantially more subjects with COPD were found when the best of either SVC or FVC was used. Forced VC tended to be higher than SVC when lung function was normal and in those with mild obstruction prior to bronchodilatation whereas SVC exceeded FVC after bronchodilatation in those who had severe bronchial obstruction.The diagnosis of COPD is highly depending on which guidelines are used for defining the disease. If FVC and not the best of SVC and FVC is used when defining COPD the diagnosis will be missed in a substantial number of patients.