The definition of what constitutes a 'normal' adrenal response to critical illness is unclear. Consequently, published studies have used a variety of biochemical criteria to define 'adrenal insufficiency'. These criteria have been based on the baseline cortisol level or the increment in cortisol following corticotropin administration. However, in critically ill patients there are a number of confounding factors that make interpretation of these tests difficult. Furthermore, in those patients who are most likely to benefit from treatment with low-dose glucocorticoids, there is no evidence that treatment should be based on adrenal function testing. In those patients in whom the diagnosis of adrenal insufficiency may be important, this diagnosis may best be made based on the free cortisol level or the total cortisol level stratified by serum albumin.