Rationale: End of test criteria can influence spirometry results. Epidemiology studies initiated before adoption of the 1987 American Thoracic Society (ATS) guidelines typically used a 1 or 2 s plateau on the volume-time curve, not a minimum test duration of 6 s, to terminate a test. Objectives: To determine the effect of changing guidelines on FEV1, FVC and FEV1/FVC during a longitudinal study of young adults. Methods: Spirometry was performed on participants in the Coronary Artery Risk Development in Young Adults (CARDIA) study. Values obtained at entry and 2, 5 and 10 years later using accepted procedures were re-evaluated using the 2005 ATS-ERS guidelines, which were in effect for the year 20 exam. Generalized estimating equations were used to adjust tests with short exhalations that were acceptable by then current end of test criteria. Results: The percentage of participants at years 0, 2, 5, and 10 with exhalations less than 6 s but with an acceptable plateau was 33%, 29%, 9%, and 2%, respectively. Exhalations less than 6 s occurred more frequently in younger and female participants, and were associated with lower FVC and higher FEV1/FVC. For short exhalations the adjusted FVC was 47 ml and 110 ml higher than the measured FVC when 6 and 8 s exhalation times were used. Conclusions: In longitudinal studies of young adults, changing end of test criteria may affect lung function, especially among younger and female participants. Determining adjusted values for tests with short exhalations may better represent the lung health of participants. © 2009 Elsevier Ltd. All rights reserved.