The effect of inhaling 0.25 and 2.0 mg of terbutaline sulphate, a beta-2-stimulant, from a metered dose aerosol was studied in five asthmatic patients during two periods of five days each. During the first period, the patients used a good spontaneous inhalation technique; during the second period, the inhalation technique was optimized and controlled. The variation of basal FEV1 and of the increase (delta FEV1) caused by 0.25 mg of inhaled terbutaline was considerable. The effect was only slightly better and the variation only slightly smaller when the controlled inhalation technique was used. The differences were not significant. In individual patients, there was no or negative correlation between delta FEV1 and the corresponding basal FEV1 value. Accordingly, the most commonly used way of expressing delta FEV1 as a percentage of basal FEV1 value was found to be insensitive. Delta FEV1, expressed as a percentage of the maximum available FEV1 increase on the same day after 2.25 mg terbutaline sulphate, was found to be most sensitive. This way of expressing delta FEV1 will increase the possibilities of detecting differences between treatments in clinical trials.