In critical illness, patients are highly stressed and should have elevated cortisol (stress hormone) secretion to adapt to stress. Dysfunction of this system is referred to as critical illness-related corticosteroid insufficiency. Free cortisol (unbound form) which is mainly responsible for its physiologic function is difficult to measure. We hypothesized that the salivary cortisol, which can be obtained by noninvasive methods, can more accurately evaluate adrenal function of critically ill patients.