Spirometry and impedance of the respiratory system were compared after increasing rates of isocapnic cold air hyperventilation (IHCA) in 10 asthmatics and 11 healthy nonsmokers. In the control group IHCA at 60% of indirect maximum breathing capacity (IMBC) resulted in a significant, equal increase in resistance at lower and higher frequencies, suggesting central airway constriction. In the asthmatics negative frequency dependence of resistance and a significant increase in resonant frequency and decrease in the reactance at 8 Hz were found at 40 and 60% IMBC. These changes suggest an increase in the resistance of the peripheral airways. The frequency dependence of resistance and resonant frequency were found to be the most sensitive measures to differentiate between normals and asthmatics. It is concluded that impedance measurements provide a suited method to assess the response in cold air provocation tests.