We compared spirometry (FEV1) and airway impedance (z) in the assessment of airway responsiveness to histamine. Airway impedance was measured by the oscillator technique during quiet breathing; both measurements were made twice after each increment of histamine during the challenge. Percentage change in impedance was related to percentage change in FEV1 according to: -delta z = 1.09-2.66 (delta FEV1), r = -0.73, p less than 0.01, ie, impedance increased on average 2.7 times as much as FEV1 fell. The cut-off point for the standard test is the histamine concentration giving a 20 percent fall of FEV1 (PC20(FEV1)). The corresponding cut-off value chosen for impedance was a 30 percent increase (PC30(z)). (PC30(z)) = 0.74 (PC20(FEV1))-0.48, rs = 0.88, where rs is the Spearman rank correlation coefficient. Thus, impedance is a more sensitive index than FEV1 because a smaller dose of histamine gave a diagnostic result. Impedance is a practical alternative to FEV1, being less arduous for the patient and requiring little cooperation.