The endocrine response to stress is complex. Elevations in the serum concentrations of the "classic" stress hormones, epinephrine and cortisol, occur following many kinds of physiologic challenge and are accompanied by elevations in corticotropin, GH, and glucagon levels. These changes are probably responsible for the hyperglycemia and hypercatabolism common to most critical illness. If volume depletion is present, vasopressin, renin, and aldosterone secretion are also likely to be stimulated. These hormones, if present in excess, may produce fluid retention and hyponatremia. In some critically ill patients, there is a dissociation of renin and aldosterone production called hyperreninemic hypoaldosteronism, but the clinical importance of this syndrome is poorly understood. Thyroid hormone metabolism is commonly affected by critical illness, which results in characteristic abnormalities of thyroid function testing known as the euthyroid sick syndrome. The reproductive axis is exquisitely sensitive to physiologic stress; hypogonadotropic hypogonadism is a common finding in critical illness. The ongoing challenge to the clinician is to determine whether seemingly abnormal hormone measurements in critically ill patients reflect an appropriate homeostatic response to severe illness or, instead, whether they denote an independent metabolic disorder that might actually cause or contribute to the patient's unstable condition. In view of the exceedingly complex (and poorly understood) interactions involved in the human response to a severe illness, a thoughtful approach to the whole patient is essential and far preferable to indiscriminate hormone testing. Such testing, at best, may be uninterpretable in light of the clinical circumstances or, at worst, may lead to therapeutic misadventures.