Chronic obstructive pulmonary disease (COPD) is a common disease with a steadily increasing prevalence and mortality. However, recent epidemiological estimates differ depending on the population studied and methods used. To investigate the prevalence, severity and burden of COPD in a primary care setting. From 4730 patients registered in a single primary care practice, all 2250 patients aged 40 years or more were invited to participate. Participants completed a questionnaire on smoking, respiratory symptoms, education and social status. A physical examination was followed by pre- and post-bronchodilator (BD) spirometry. Of the eligible patients, 1960 (87%) participated. 92% of spirometric tests met the ATS criteria. Airflow limitation was demonstrated in 299 (15%) of the participants pre-BD and in 211 (11%) post-BD. COPD was diagnosed in 183 patients (9.3%). Of these patients, the degree of post-BD airflow limitation was mild in 30.6%, moderate in 51.4%, severe in 15.3% and very severe in 2.7%. Only 18.6% of these patients had previously been diagnosed with COPD; almost all of these had severe or very severe airflow limitation. As a result of the study, a diagnosis of asthma was made in 122 patients. The prevalence and underdiagnosis of COPD in adult patients in this primary care setting made case finding worthwhile. Large numbers of newly detected patients were symptomatic and needed treatment. Limiting investigations to smokers would have reduced the number of COPD diagnoses by 26%.