We wanted to assess compliance with regular pulmonary medication, in 156 patients in a general practice setting. Patients completed a questionnaire on their reported daily intake, the perceived prescription, and their self-assessed perceived compliance with pulmonary medication. Compliance was defined as a reported daily intake > or = 50% of the prescribed amount. Using this definition, 30% of patients were considered to be compliant. Compliant patients had daily symptoms more often, were more often prescribed two or more different medications, and a greater proportion of them had at some time consulted a chest physician. When patients correctly perceived the prescription to refer to medication which had to be taken on a regular basis, they were more likely to be compliant. We conclude that, since less than one third of patients was compliant with medication, more efforts are needed to improve compliance in general practice. Non-compliance with medication may well provide an alternative explanation for the discrepancy between prescribed medication and medical outcome, which has been labelled in the literature as "undertreatment".