Consecutive spirometry tests from three hospitals at two academic medical centers between December 1, 2003, and February 29, 2004, were reviewed. Results were taken from the University Hospital at The Ohio State University, and from Indiana University Hospital and Wishard Memorial Hospital, both of which were a part of the Indiana University Medical Center. All PFTs were performed and reported with the goal of meeting ATS standards for acceptability and reproducibility.11 Tests were reviewed, and those that were felt to be inaccurate or uninterpretable were excluded from analysis. FEV1, FVC, and FEV1/FVC ratio were entered into a database (ACCESS; Microsoft; Redmond, WA). Demographic features were also recorded, including age, race (self-reported), sex, height, and weight. Because no protected health information was recorded and the data were obtained from an existing database, the institutional review boards at both institutions approved this study as exempt research, and patient consents were not obtained. Predicted and percent predicted values were calculated for FEV1, FVC, and FEV1/FVC ratio using reference values from Hankinson et al (the Hankinson data set),,8 Crapo et al (the Crapo data set),12Knudson et al (the (the Knudson data set),13and Morris et al (the Morris data set).14 In addition, the values for LLN of the FEV1/FVC ratio were calculated for the Hankinson and Crapo data sets using their published equations. Airflow obstruction was diagnosed by a fixed FEV1/FVC ratio of < 75% or 70%, or by finding an FEV1/FVC ratio less than the predicted LLN derived by the methods of Hankinson et al,8 or Crapo et al,8 or by having an FEV1/FVC ratio of < 88% predicted for men and < 89% predicted for women. For severity interpretation, the results for FEV1 and FVC were race-corrected for African-Americans by 12% for the data sets of Knudson, Morris, and Crapo, based on ATS recommendations. The Hankinson data set has separate prediction equations for ethnicity/race, which were used in this study, and does not require a separate correction factor for African Americans.