Severe chronic adrenal insufficiency (primary or secondary) is a potentially lethal disorder, unless the patient is regularly substituted with glucocorticoids, usually with hydrocortisone (15-25 mg/day) and with 9 alpha-fluor-hydrocortisone (0.05-0.2 mg/day) in addition in patients with the primary adrenal disorder (Addison's disease). In stressful situations and in febrile disorders, the glucocorticoid dosage must be increased prophylactically in order to prevent an "adrenal crisis". Most women with adrenal insufficiency will profit from the additional substitution of dehydroepiandrosterone (DHEA) with regard to well-being and sexual function. A patient with acute adrenal insufficiency will die if the diagnosis is missed and high-dose glucocorticoid treatment is not instituted immediately. Acute adrenal insufficiency developing de novo in an intensive care patient (e.g. from adrenal hemorrhage or adrenal vein thrombosis) is a most challenging diagnosis. In these patients, however, survival not only depends on glucocorticoid substitution but also on the underlying disease.