The aim of this prospective study was to detect subjects in the general population with objective signs of chronic obstructive pulmonary disease (COPD) or asthma at an early stage. This was done by means of a two-stage protocol involving screening and a subsequent 2-yr monitoring of all subjects with positive results of screening. The study was done in 10 general practices located in the eastern part of the Netherlands. A random sample was taken from the general population aged 25 to 70 yr. All known COPD and asthma patients were excluded. A total of 1,749 subjects met the inclusion criteria: 1,155 subjects (66%) agreed to participate in the screening stage of the study. A total of 604 subjects (52.3%) showed symptoms or objective signs of COPD or asthma during the screening and were considered "positive." Of those with positive screening results, 384 subjects (64%) agreed to participate in the second, 2-yr monitoring stage of the study. The costs involved in detection were calculated for three different scenarios, as follows: (1) The detection of subjects with persistently decreased lung function or an increased level of bronchial hyperresponsiveness (BHR) during 6 mo of monitoring; (2) Scenario 1 plus the detection of subjects with a rapid decline in lung function with signs of BHR during 12 mo of monitoring; (3) Scenario 2 plus the detection of subjects with a moderate increase in the decline in lung function or signs of BHR during 24 mo of monitoring. The costs of lung function assessments, organization, transportation, and patient time were included. The costs were converted to United States dollars on the basis of purchasing power (1 United States dollar = 2.08 Netherlands guilders). During the second stage, 252 subjects were detected with objective signs of COPD or asthma at an early stage. Smoking status as a screening criterion was neither sensitive nor specific. Because there was no evidence of biased recruitment or selection during the program, the proportions of subjects found to have objective signs of COPD or asthma at an early stage could be extrapolated to the general population. Of the general population, 7.7% showed persistently reduced lung function or increased BHR. Another 12.5 % of the general population showed a rapid decline in lung function (> 80 ml/yr) in combination with signs of BHR, and a further 19.4% of the general population showed mild objective signs of COPD or asthma. The average costs per detected case varied from US$953 (Scenario 1) to US$469 (Scenario 3). In conclusion, detection of COPD or asthma at an early stage by means of a two-stage protocol was feasible at relatively little expense in comparison with other mass screening programs. Persistently decreased lung function or a rapid decline in lung function (Scenario 2) was observed in approximately 20% of the general adult population.