In order for an individual FVC maneuver to be considered acceptable according to the 1994 American Thoracic Society (ATS) standards it must meet end of test (EOT) criteria of a minimum expiration time of 6 s with minimal volume change (0.03 L) over 1 s. We have found that while these criteria are often not met in children, most of the tests do meet the ATS criteria for reproducibility with repeated efforts. We, therefore, sought to develop new EOT criteria that would be more appropriate for children and in keeping with the findings for reproducibility. Using an exponential curve fitting of the volume time tracing, we determined a theoretical curve that closely approximated the actual curve (mean difference between actual and derived FEV1 0.01 +/- 0.04 L). The theoretical FVC (FVCFULL) at the point where the curve reached its asymptote was then determined using the fitted curve. Since this point could be difficult to reach for some patients, 95% of FVCFULL (FVC95) was proposed as the new EOT in children. Data from 382 patients were reviewed. Their ages ranged from 5 to 18 yr and their FEV1s from 21 to 120% of predicted. Only 19% of the patients met current ATS EOT requirements despite the fact that 91% met the reproducibility criteria for both FEV1 and FVC. However, 90% of them reached their FVC95. When this was broken down by age, 37% of those < or = 7 yr failed to reach FVC95 whereas only 4% of the older children failed to do so. It is proposed that patients be coached to obtain maximal effort and that the curves be individually fitted to an exponential equation to determine whether the patient has achieved EOT as defined by FVC95.