OBJECTIVE: To determine medication possession ratio (MPR) of patients with asthma/COPD drugs. METHOD: Individual patient's volumes of asthma/COPD drugs (ATC-code R03) for 2000-2004 were obtained from a pharmacy record database. For each patient the MPR was calculated as the percentage of the treatment time that the patient had drugs available. MAIN OUTCOME MEASURE: Medication possession ratio (MPR). RESULTS: A total of 1,812 patients, 20 years and older, with dispensed asthma/COPD drugs were registered in the database, 928 patients (51%) had acquired drugs less than once per year (68% for 20-29 years old) during the 5-year study. Only 13% of the patients had steroids and steroid combinations available to allow continuous treatment. Eight percent of the patients 20-29 years old had MPR > or = 80% of all the included drugs and 5% when only steroids and steroid combinations were considered. About 25% of the patients had acquired 80% of the total volume of asthma/COPD drugs. CONCLUSION: The majority of the patients, and particularly those in the youngest age group used asthma/COPD drugs only sporadically. This may indicate undermedication which is likely to have a negative impact on patient outcome.