Summary The aim of this exploratory study was to investigate associations between sick leave in workers with asthma or COPD and disease-related variables, psychosocial variables, and work characteristics. Hundred and eighty-nine patients with physician-diagnosed asthma ( N = 118 ) or COPD ( n = 71 ) who had paid work in the past 12 months completed questionnaires on sick leave, health complaints, functional limitations, work characteristics and psychosocial issues, and underwent a pulmonary function test (FEV 1 and FVC before and after bronchodilation). Logistic regression analyses were performed to investigate variables independently associated with high sick leave (i.e. more than twice a year and/or longer than 1 month per episode). Asthma patients, not having an emotionally difficult job, with low job satisfaction, who had changed employers, utilized job control, and who encountered pulmonary aggravating factors at work were found to have a higher incidence of sick leave. COPD patients, who had informed the employer or colleagues about the disease, who did not have difficult tasks at work, who did not hide dyspnea and limitations, and who reported high fatigue were showing higher sick leave. FEV 1 and FVC were not associated with sick leave in either group. It was concluded that psychosocial variables, work characteristics, functional limitations, and complaints play a more important role in sick leave in workers with asthma and COPD than FEV 1.