The prevalence of chronic obstructive pulmonary disease (COPD) in the elderly is poorly known. The aim of this study was to determine the prevalence of COPD and to analyse the factors associated with it in an elderly Finnish population. The population consisted of persons resident in Lieto, Finland, born in or before 1926. One thousand one hundred and ninety-six individuals (488 men and 708 women, 93% of those eligible) participated in the survey, which was carried out in 1990-91 at the Lieto Health Centre. The methods included a nurse's interview and spirometry with a bronchodilator test. The participants were also clinically examined, and the number of subjects with COPD was determined using simple diagnostic criteria. Sixty-one men and 21 women with COPD were found. Thus the prevalence was 12.5% for the men and 3.0% for the women. In both sexes only about 2% of those who had never smoked suffered from COPD, but among the current smokers the prevalence was 35% for the men and 13% for the women. In a stratified analysis COPD was commonest among those with a low social status and histories of smoking and working in dusty occupations. In a majority the obstruction was only minimally reversible when tested with salbutamol inhalation. The study confirmed that the prevalence of COPD in the elderly with negative histories of smoking is low, and emphasizes the importance of reducing smoking as the only effective preventive measure.