1. In five subjects, ten consecutive forced expiratory spirograms were recorded on each of 6 different days and the total forced expiratory time (FET) on each spirogram was measured. The mean overall coefficient of variation of the FET was 11·3% and of the ratio forced vital capacity (FVC)/FET was 8%.
2. Twenty-two subjects with probable small (<2 mm diameter) airway dysfunction as manifested by a reduced (<75% predicted) maximum mid-expiratory flow (MMEF) in the middle 50% of the FVC with normal FVC and FEV1 (>80% predicted) were studied before and after bronchodilatation. There was no significant correlation between change in FET and change in MMEF (r = 0·36, P > 0·01), but between change in MMEF (ΔMMEF) and change in FVC/FET (ΔFVC/FET) there was a positive correlation (r = 0·78, P < 0·001).
3. In six of these twenty-two subjects, the ΔFVC/FET and ΔMMEF on bronchodilatation correlated well with the change in residual volume (ΔRV), but neither measurement correlated well with the change in the ‘closing volume' measurement expressed as the ratio of phase 4/VC(%). The ΔFET did not correlate well with either ΔRV or Δ[phase 4/VC(%)].
4. In eighty-one selected normal subjects (FVC, FEV, and MMEF all >80% predicted) there was better correlation of age with the FVC/FET (r = −0·676) than with the FET (r = +0·43); both correlations were significant (P < 0·001).
5. These results do not support the use of the FET as a suitable test of small airway function; however, the mean flow rate expressed as FVC/FET may, in the presence of a normal FEV1/FVC ratio, be an adequate test of small airway function.