The clinical significance of Iso-volume FEF25-75% in assessing airway responsiveness to an inhaled bronchodilator was evaluated in 167 asthmatics who presented with variable degrees of airway dysfunction. Iso-volume FEF25-75% identified responsiveness in one patient (5%) out of 20 of the most dysfunctional asthmatics, in three (20%) out of 15 of the least dysfunctional asthmatics and over-all in four (8%) out of 47 asthmatics not showing significant responsiveness via the FEV1. Over-all, FEV1 identified significant responsiveness in more patients (68.6%) than did the Iso-volume FEF25-75% (61%). The authors conclude that the Iso-volume FEF25-75% adds little to the FEV1 in the assessment of obviously dysfunctional asthmatics, the FEF25-75% as conventionally measured is a relatively useless post-bronchodilator measurement and that the FEV1 remains the single best spirometric test with which to assess airway responsiveness in asthmatics.