Measurement of respiratory system input impedance (Zrs) by forced oscillation (FO) has generally been limited to frequencies less than or equal to 50 Hz, and correlations with spirometry have been variable. Using FO from 4 to 256 Hz in normals, Jackson and colleagues recently described a first acoustic antiresonance frequency (Far,1) at approximately 170 Hz. Using the same frequency range, we compared several Zrs spectral characteristics with spirometry in 12 chronic airflow obstruction (CAO) patients (range FEV1 0.8 to 2.0 L) and 10 matched controls. Compared with controls, patients had a higher first resonance frequency (Fr,1) (mean +/- SD = 15 +/- 5 versus 10 +/- 2 Hz, p less than 0.02) and a higher Far,1 (196 +/- 11 versus 172 +/- 13 Hz, p less than 0.0002). Good correlations occurred between % predicted FVC and the Far,1 (r = -0.81, p less than 0.0000), between FEV1/FVC and the reactance at 20 Hz (r = -0.6, p less than 0.003), between FEV1 and Far,1 (r = -0.74, p less than 0.0001). Because Far,1 may be affected by airway wall mechanical properties, the shift in Far,1 seen in these patients may be due to airway wall properties in CAO. We conclude that measurement of Zrs up to 256 Hz requires little patient cooperation and may be clinically useful. It can differentiate CAO patients from controls and correlates well with spirometry. The first acoustic antiresonance frequency may reflect airway mechanical properties and provide information not available from Zrs measured at lower frequencies.