Synthetic corticosteroids are frequently used to manage asthma and other inflammatory diseases. The timing of such drugs (whether ingested, inhaled, or infused) in relation to body rhythms influences the magnitude of both desired and undesired effects. It is crucial that corticotherapy be correctly scheduled to the circadian system of the hypothalamic-pituitary-adrenocortical (HPA) system. The secretion of cortisol from the adrenal cortex is not constant during each 24-hour period. Instead, production of this hormone varies as a high-amplitude circadian rhythm, with most of the secretion taking place during the initial hours of the activity span and very little late in the evening and during the first half of the sleep span. Results of laboratory and human studies indicate that the timing of exogenous corticosteroids, in relation to the circadian rhythm in HPA activity, is a critical factor. For example, the optimization of corticosteroid therapy for asthmatics entails daily (or alternate-day) administrations in the morning and, if necessary, early afternoon. By timing exogenous corticosteroids early during the activity span, the risk of adrenal suppression is minimized or avoided while bronchial patency is optimally enhanced, i.e., increasing the 24-hour average forced expiratory volume in 1 second (FEV1) and reducing its nocturnal dip. Clinical findings indicate that these results are obtainable with both acute and chronic corticosteroid therapies. In contrast, splitting the daily dose of corticosteroids into several small administrations, such as at mealtimes and before bedtime, markedly increases the likelihood of adrenal suppression without achieving the desired therapeutic effect. The dosing of synthetic corticosteroids late in the afternoon or evening, whatever the route of delivery, suppresses pituitary adrenocorticotropic hormone (ACTH) production during subsequent 24-hour spans, resulting in adrenocortical inhibition. Also, morning dosing of corticosteroids over many years seems to induce less--if any--osteopenia compared to dosing at other times. The adrenal response to exogenous administration of ACTH also is circadian-rhythmic. ACTH dosing in the morning results in greatest adrenal response in terms of cortisol secretion, while dosing in the evening results in least response. Knowledge of the circadian organization of the HPA axis is necessary to optimize the effect of synthetic corticosteroids, whether they be used to treat asthma, rheumatoid arthritis or other cortico-dependent diseases, or as a substitution therapy for Addison's disease.