Twenty-eight asthmatics inhaled an isoprenaline aerosol on several occasions. The dose administered was constant for each subject on each occasion, yet the responses of each subject measured spirometrically differed on most of the separate occasions. The individual response patterns showed the following features. Spontaneous worsening of the asthma was associated with spirometric responses which in some cases increased directly and predictably with the corresponding fall in pre-treatment FEV1. In other cases the increase in response was less regular and in a few it bore no relation whatsoever to the pre-treatment FEV1. In three subjects a response occurred when the pre-treatment FEV1 was greater than the predicted value. There were variations between subjects in the magnitude of increase in spirometric response to inhaled bronchodilator occurring when there was a fall in pre-treatment FEV1. The great majority of subjects never achieved their predicted FEV1 after maximal doses of isoprenaline. The study of individual response patterns is of value in the selection of subjects for comparative trials of different bronchodilator drugs. The present study draws attention to an irreversible component in the reduction of airways calibre that occurs in asthma.