Objective: Our goal is to determine short-term intraindividual biologic and measurement variability in spirometry of patients with a wide range of stable chronic obstructive pulmonary disease severity, using datasets from the National Emphysema Treatment Trial (NETT) and the Lung Health Study (LHS). This may be applied to determine criteria that can be used to assess a clinically meaningful change in spirometry. Methods: A total of 5,886 participants from the LHS and 1,215 participants from the NETT performed prebronchodilator spirometry during two baseline sessions. We analyzed varying criteria for absolute and percent change of FEV 1 and FVC to determine which criterion was met by 90% of the participants. Results: The mean Ϯ SD FEV 1 for the initial session was 2.64 Ϯ 0.60 L (75.1 Ϯ 8.8% predicted) for the LHS and 0.68 Ϯ 0.22 L (23.7 Ϯ 6.5% predicted) for the NETT. The mean Ϯ SD number of days between test sessions was 24.9 Ϯ 17.1 for the LHS and 85.7 Ϯ 21.7 for the NETT. As the degree of obstruction increased, the intersession percent difference of FEV 1 increased. However, the absolute difference between tests remained relatively constant despite the severity of obstruction (0.106 Ϯ 0.10 L). Over 90% of participants had an intersession FEV 1 difference of less than 225 ml irrespective of the severity of obstruction. Conclusions: Absolute changes in FEV 1 rather than percent change should be used to determine whether patients with chronic obstructive pulmonary disease have improved or worsened between test sessions. Keywords: forced expiratory volume; obstructive lung diseases; reproducibility of measurements; spirometry; vital capacity (Received in original form June 24, 2005; accepted in final form February 23, 2006 ) Supported by contract N01-HR-46002 from the Division of Lung Diseases, National Heart, Lung, and Blood Institute, and National Institutes of Health (The Lung Health Study). The National Emphysema Treatment Trial is supported by contracts with the National Heart, Lung, and Blood Institute (N01HR76101, N01HR76102, N01HR76103, N01HR76104, N01HR76105, N01HR76106, N01HR76107, N01HR76108, N01HR76109, N01HR76110, N01HR76111, N01HR76112, N01HR76113, N01HR76114, N01HR76115, In patients with chronic obstructive pulmonary disease (COPD), spirometry, including FEV 1 and FVC, is a widely used measure of progression of disease and response to treatment. According to past American Thoracic Society (ATS) standards for spirometry, within-session reproducibility was considered to be 200 ml or a 5% change in FEV 1 , whichever is larger (1), based on data from the Third National Health and Nutrition Examination Survey (NHANES III) (2). The current proposed ATS/European Respiratory Society guidelines for spirometry standardization recommend within-session reproducibility within 150 ml for FEV 1 and FVC (3). Small previous studies suggested that patients with COPD may have greater variability within and between spirometry test sessions than do individuals with normal lung function (4). Previous articles have...