Public interest needs to be focused on the economic burden of asthma on society because of the increasing prevalence of the condition. Asthma is common in individuals of working age and sick leave is an important health-economic issue. In the present study we looked at the prevalence of asthma in a sick leave register. Individuals on sick leave due to asthma, individuals on sick leave due to any other respiratory disorder or symptom and individuals on sick leave due to non-respiratory conditions were included in a questionnaire based study. Individuals in the register diagnosed with asthma could be classified as current asthmatics or possible asthmatics in respectively 94% and 99% of the cases. They were also ex-smokers to a greater extent than the other groups, which was more pronounced in males. However, individuals on sick leave due to 'any other respiratory disorder' could be classified as current asthmatics or possible asthmatics in respectively 19% and 30% of the cases. The corresponding figures in the group on sick leave due to 'other diagnoses' were 7% and 10%. Hence, there is evidence that asthma is an under-reported diagnosis and this must be taken into consideration when sick leave registers are used in health-economic studies.