Corticosteroids are of key importance in controlling the immune system and in the maintenance of cardiovascular function. Thus, an adequate function of the adrenal cortex is essential for survival in critical illness. There is growing evidence that adrenocortical function can become impaired during critical illness, because of deleterious effects of cytokines, leading to a state of relative adrenal dysfunction. Under these circumstances, administration of corticosteroids is necessary for recovery of the patient. How such a state of adrenal dysfunction should be detected is still not clear, however, making it difficult to decide when to administer corticosteroids and to which patients. (C) 2000 Lippincott Williams and Wilkins.