The sensitivity and reliability of a new index of flow volume spirometry, area under the expiratory flow-volume curve (AFV), was tested in metacholine challenges in 42 patients with bronchial hyperreactivity. The responses of AFV were compared with those of VC, FVC, FEV1, PEF and MEF50. After inhaled metacholine AFV showed the greatest decrease (36.0%) from the baseline; the corresponding changes of VC, FVC, FEV1, PEF and MEF50 were 12.9%, 15.9%, 20.9%, 20.3%, and 33.3%, respectively. The changes expressed in units of coefficient of variation (variation between two successive curves) were: VC 2.9, FVC 3.7, FEV1 3.7, PEF 2.9, MEF50 2.9 and AVF 6.7. Thus, AFV seemed to be the most sensitive indicator of bronchoconstriction in metacholine challenges tests. In addition, the changes of AFV correlated well with those of FEV1 (r = 0.939, p less than 0.001).