Corticosteroids have now been used in the treatment of bronchial asthma for about 30 years but objective physiological evidence of their effects in this condition has only become available over the past 8 years. Studies with oral prednisolone, intravenous prednisolone and hydrocortisone, as well as with inhaled budesonide, in patients with chronic bronchial asthma, have shown that there is a time lag between the administration of these drugs and the onset of improvement in the patients' conditions. This time lag is even longer when corticosteroids are given to patients with severe asthma. It appears that these drugs exert an effect on both the central and the peripheral airways. Unlike bronchodilators corticosteroids do not appear to increase the degree of ventilation perfusion mismatching present in asthma. They have been shown to decrease the alveolar-arterial oxygen tension gradient and the venous admixture effect with a consequent rise in arterial oxygen tension. It is still uncertain how corticosteroids work in asthma; it seems, however, that one of the ways may be through their effects on the beta adrenergic receptors.