Several studies have shown that up to 75% of patients with chronic obstructive pulmonary disease (COPD) remain undiagnosed. Early diagnosis of such individuals will allow appropriate preventive and therapeutic measures to be prescribed. The PADOC project aimed to determine the efficacy of a COPD screening program for Spanish primary care settings. The participating primary care physicians were required to administer a spirometric test (forced expiratory volume) to all individuals who had not previously been given a diagnosis of COPD, who visited the clinic for any reason over a period of three months and who met the following enrollment criteria: a) age over 35 years and b) smoker of more than 10 cigarettes per day or ex-smoker of more than 10 packs per year. Individuals meeting the criteria for suspicion of COPD (FEV1 < 90% of predicted and FEV1/FVC < 70%) were referred to a pneumologist for confirmation of the diagnosis. One hundred ninety-four primary care physicians participated, administering 3,209 valid spirometric tests. The physicians identified 723 likely cases of COPD (22.5%). Pneumologists examined 278 patients (38.4% of the likely cases) and the final diagnosis was COPD in 153 of the 278 (55%) and asthma in 28 (10%). Therefore, 4.3% of all patients given spirometric tests received a diagnosis of COPD and 0.8% received a diagnosis of asthma. Agreement between spirometric measurements taken by the primary care physicians and those taken by pneumologists was low for FVC and FVC (%) (intra-class correlation coefficient ICC = 0.38 and 0.45, respectively) but good for FEV1 and FEV1 (%) (ICC = 0.78 and 0.67, respectively). We conclude that primary care screening for COPD is possible and would allow us to detect up to 22% of possible cases. Patient flow from one level of clinical care to another should be improved, given that most of the possible cases detected (61.6%) were not seen by the referral pneumologist. Agreement between spirometric measurements taken by primary care physicians and pneumologists was low for FVC but good for FEV1.