The American Thoracic Society recommends that the largest FEV1 be reported from a set of forced expiratory vital capacity maneuvers performed with maximal expiratory effort. However, increased expiratory effort can decrease the FEV1. When we evaluated the peak expiratory flow rate (PEFR) in 5 normal subjects, measured from flow-volume curves, as a noninvasive index of expiratory effort, it was positively correlated with indices of effort obtained by using an esophageal balloon. We then measured the difference (dFEV1)between the largest FEV1 and FEV1 from the maneuver with the highest PEFR during 10 test sessions in 10 normal subjects. Thus, dFEV1 was always greater than or equal to 0. The mean dFEV1 was 110 ml for all sessions but decreased to 80 ml when maneuvers with poorly reproducible PEFR or forced expiratory vital capacity values were discarded. We also reviewed 9.471 spirometry sessions from outpatients and found dFEV1 to be greater than 50 ml in 26% of this population and greater than 151 ml in 7%. We concluded that during standard spirometry, FEV1 is inversely dependent on effort. Maximal effort decreases FEV1 because of the effect of thoracic gas compression on lung volume. We recommend that values from spirometry maneuvers that demonstrate submaximal effort, indicated by a decreased PEFR, be discarded. The flow-volume curve display of superimposed efforts facilitates the recognition of submaximal efforts.