We have assessed the value of the area under the MEFV curve (AUC) as an index of respiratory function in chronic bronchitis and compared it with PFR, FEV1, FVC, volume at 75% PFR (V75), V50, V25, F50 and F75. The reproducibility of these parameters was tested in 10 normal subjects and 10 patients with chronic bronchitis. The FVC was the most reproducible while the coefficient of variation for the AUC was the same as for the other MEFV curve indices. The sensitivity (percentage change on bronchodilatation after intravenous aminophylline) of the above measurements was also tested in a further nine patients with chronic bronchitis. The AUC was much more sensitive to bronchodilatation than any of the other measurements. Therefore although the AUC was less reproducible than simple spirometric indices, it was more sensitive to bronchodilatation by a greater factor. This probably outweighs its poor reproducibility and AUC would therefore seem to be a useful new index of bronchodilatation in chronic bronchitis.