The upper airways may contribute to increases in airway resistance in response to a bronchial challenge, and thus decrease the specificity of such challenge tests to diagnose airway hyperresponsiveness when forced oscillation techniques are used to evaluate changes in respiratory system resistance (Rrs). A concomitent decrease in respiratory system reactance (Xrs) may indicate a change in the intrathoracic airways and/or lung parenchyma, provided that extrathoracic airway wall motion is prevented. To test the value of Xrs in the evaluation of bronchial hyperresponsiveness, we studied the respiratory impedance response to methacholine in 38 children with a history of asthma (aged 6-14.5 years), and compared the results to changes in the forced expiratory volume in one second (FEV1). Rrs and Xrs were measured by the forced oscillation technique with pseudorandom (11 subjects) or sinusoidal (27 subjects) pressure variations applied around the child's head to minimize upper airway wall motion. Changes in Rrs and in Xrs at 12 Hz (Rrs12, Xrs 12) correlated significantly with changes in FEV1 (P < 0.005). A decrease in FEV1 > or = 20% was observed in 23 subjects. When these 23 subjects were compared with the 15 children who did not show significant changes in FEV1, the responding group had larger mean +/- SEM changes in Rrs (116.0 +/- 13.2% vs 60.4 +/- 11.4%, P < 0.006) and in Xrs (-2.1 +/- 0.4 hPa.s/L vs -0.9 +/- 0.3 hPa.s/L, P < 0.03) than the nonresponders. The receiver operating characteristics (ROC) curve analysis was used to assess the diagnostic value, i.e., specificity and sensitivity, of different levels of change in Rrs and Xrs, with reference to FEV1. The overall incidence of false results was similar for Rrs and Xrs. The optimum diagnostic value for Rrs was a 70% increase, which corresponded to a sensitivity of 87% and a specificity of 67%. For Xrs the optimum decision level was -1 hPa.s/L, corresponding to a sensitivity of 70% and a specificity of 80%. It is concluded that Xrs may improve the specificity of the forced oscillation technique in interpreting the airway response to methacholine. This may be of particular interest in young children unable to perform forced expirations.