Short term variability in FEV1 and responsiveness to inhaled bronchodilator were measured in 150 patients with obstructive ventilatory defects. The range of initial FEV1 was 0 5-4 71 and the natural variability over a 20 minute period when expressed in absolute terms was similar over the entire range, and differed insignificantly from that found in normal subjects. The increase in FEV1 and vital capacity (VC) required to exclude natural variability with 95% confidence in these patients was 160 ml and 330 ml respectively. Natural variability when expressed in percentage terms was negatively correlated with the level of FEV1 recorded. The analysis of changes in FEV1 and VC after administration of bronchodilator used absolute and percentage criteria for response.

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