Adrenal insufficiency is a challenging problem for critical care physicians. Failure to recognize its presence and to institute appropriate therapy when the patient is ill enough to require intensive care can lead to a life-threatening situation. This review covers the circumstances under which adrenal insufficiency should be suspected, the steps necessary to document adrenal dysfunction, and the appropriate therapy. Normal Response of the Adrenal Glands to Stress Measurement of Cortisol Concentrations in Stress.Definition of the normal adrenal response to a variety of stresses has been established by a number of studies. Shortly after it became possible to measure glucocorticoid levels, initial determinations showed plasma cortisol concentrations to be 4 to 5 times above normal in moribund patients [ 1 These patients, moreover, did not experience the usual increase in cortisol levels after administration of adrenocorticotrophic hormone (ACTH). Similar elevated plasma concentrations of cortisol were reported in patients suffering from septic shock [2] and in dogs given endotoxin [3]. In the study by Melby and colleagues [3], the importance of intact hypophyseal function for the stress-associated increases of cortisol was established, because no increase occurred if dogs had been hypophysectomized. Other acute stresses, such as those caused by severe trauma [4], sepsis without shock [5], and surgical procedures [6,7], are associated with 2to 3-fold increases in plasma cortisol concentrations. The stress from surgical procedures is so powerful that the increase in cortisol cannot be prevented by the concomitant administration of exogenous glucocorticoid steroid [7], which under ordinary circumstances would suppress endogenous glucocorticoid production. Similar cortisol increases usually occur in infections with fever [8][9][10][11] and may also be accompanied by a loss in normal circadian rhythm [9] that is characteristic of corticosteroids. Occasional instances of normal plasma cor-