Chronic obstructive pulmonary disease (COPD) is often misdiagnosed as asthma, leading to inappropriate treatment and suboptimal patient outcomes. As part of a prospective study of patients with a history consistent with obstructive lung disease, we compared prior diagnostic labels with a study diagnosis based on spirometric results. We enrolled persons 40 years of age or older with prior diagnoses or medications consistent with obstructive lung disease. Patients were recruited via random mailing to primary care practices in Aberdeen, Scotland, and Denver, Colorado. Prior diagnoses of chronic bronchitis or emphysema (CBE) and asthma were reported by the subjects. Participants underwent pre- and post-bronchodilator spirometry. A study diagnosis of COPD was defined using post-bronchodilator forced expiratory volume in 1 second/forced vital capacity (FEV(1)/FVC) < 0.70. Spirometric examination was complete in 597 patients, of whom 235 (39.4%) had a study diagnosis of COPD. Among subjects with a spirometry-based study diagnosis of COPD, 121 (51.5%) reported a prior diagnosis of asthma without concurrent CBE diagnosis, 89 (37.9%) reported a prior diagnosis of CBE, and 25 (10.6%) reported no prior diagnosis of obstructive lung disease. Despite the availability of consensus guideline diagnostic recommendations, diagnostic confusion between COPD and asthma appears common. Increased awareness of the differences between the two conditions is needed to promote optimal patient management and treatment.