SETTING: Chronic bronchitis and chronic obstructive pulmonary disease (COPD)/emphysema occur frequently among middle-aged and elderly asthma patients who smoke. OBJECTIVE: To test how much this comorbidity increases the use and costs of health services in comparison with asthma alone. DESIGN: A sample of 6000 adults with a clinical diagnosis of asthma was extracted from a nationwide health insurance register for a postal inquiry. Comorbidity and the use of health services were measured using a questionnaire. Data on medication expenses were obtained from the national prescription register. RESULTS: Altogether 4956 individuals replied, of whom 3160 asthma patients aged >/=40 years (response rate 85%) were chosen for this investigation. Asthma patients with COPD/emphysema (12% of the series) accounted for 21% of all doctor consultations, 39% of the total number of hospital in-patient days and 27% of the total expenses, of which one third were medication costs. The mean annual gross expenditure on treatment services and anti-asthma medications was euro754 per patient for those with asthma alone and euro2107 for those with concurrent COPD/emphysema. Current smoking further increased costs among COPD patients. CONCLUSION: To prevent pulmonary comorbidity and the related high costs, cessation of smoking should be an integral part of the treatment provided for asthma patients.