Systemic absorption of inhaled corticosteroids taken in high doses (> or = 1500 micrograms beclomethasone dipropionate or budesonide daily), may cause suppression of the hypothalamo-pituitary-adrenal axis. Patients taking long-term high dose inhaled steroid therapy might therefore be at risk of adrenal crisis at times of stress. Plasma cortisol levels were measured in 24 adults with severe acute asthma who had not received treatment with systemic corticosteroids prior to hospital attendance. Seven were not taking inhaled steroids, four were taking 600-1200 micrograms and 13 were taking 1500-2400 micrograms beclomethasone dipropionate or budesonide daily. Plasma cortisol levels in these 13 (median 594 nmol l-1, interquartile range 399-620 nmol l-1) were similar to levels in those taking lower dose/no inhaled steroids (median 512 nmol l-1, interquartile range 287-1050 nmol l-1): there was no relationship between inhaled steroid dose and cortisol level. Nine of the 24 patients failed to achieve plasma cortisol values > 500 nmol l-1 (the normal response to an insulin stress test). When compared with the remaining 15, they had less severe asthma as indicated by higher arterial oxygen tension (P < 0.01) and peak expiratory flow (P < 0.03). Patients taking long-term high dose inhaled corticosteroids appear to be able to mount an appropriate adrenocortical response to the stress of severe acute asthma.