Although hypothalamic-pituitary-adrenal (HPA) axis suppression has traditionally been viewed as an adverse event after long-term administration of corticosteroids, this effect can also be used to compare the potency of different inhaled corticosteroids. However, various factors such as the dose, frequency of administration, treatment duration, study population (patients with asthma versus normal volunteers), and prior systemic steroid therapy influence adrenal suppression with inhaled corticosteroids. The different adrenal function tests available and the results produced with these tests also must be considered along with the clinical relevance of such results. Whereas low doses of inhaled corticosteroids are likely to cause minimal or no HPA axis suppression, long-term high-dose inhaled corticosteroid use may result in significant suppression by effectively replacing endogenous steroid production. The risk of acute adrenal insufficiency in patients taking low/medium-dose inhaled corticosteroids is minimal, but patients receiving long-term high-dose treatment may require supplementary systemic steroids during stress challenges, especially if they have previously received long-term systemic steroid treatment.