The objective of this study was to evaluate the definitions for classification of chronic obstructive pulmonary disease (COPD) recommended by the American Thoracic Society (ATS) and the Global Initiative for Chronic Obstructive Lung Disease (GOLD). Using data from the U.S. population-based third National Health and Nutrition Examination Survey (NHANES III), we compared the number of individuals in the U.S. population who met definitions of airflow obstruction based on the fixed ratio of FEV /FVC < 0.70 criterion and on the ATS lower 95% confidence limit (lower limits of normal LLN) criterion. Further, we evaluated the definitions in the context of physician-diagnosed obstructive airways diseases and respiratory symptoms. In comparison to the FEV1/FVC < LLN and FEV1 < 100% predicted definition, the fixed ratio-based definition for mild COPD underestimates airflow obstruction by 29% in 20-49-year-olds and overestimates it by 58% in 50-80-year-olds. In comparison to the FEV1/FVC < LLN and FEV1 < LLN definition, the fixed ratio-based definition for moderate COPD underestimates airflow obstruction by 31% in 20-49-year-olds and overestimates it by 37% in 50-80 year olds. Based on our estimation, approximately 0.9 million (26%) of symptomatic individuals out of the 3.6 million U.S. adults aged 20-49 years who have airflow obstruction (FEV1/FVC < LLN and FEV1 < LLN definition) may have undiagnosed respiratory disease. In conclusion, using the FEV1/FVC < 0.70 criterion will substantially under-diagnose airway obstruction in younger individuals and substantially over-diagnose COPD in older individuals.