A reduced secretion of cortisol has been proposed as a possible explanation of the symptoms in chronic fatigue syndrome. However, the evidence of hypocortisolism in chronic fatigue syndrome is conflicting. In order to simultaneously assess possible alterations in adrenocortical sensitivity and secretory adrenal reserve, the authors administered both low-dose and high-dose ACTH to a group of 18 chronic fatigue syndrome patients and 18 age- and gender-matched healthy comparison subjects. No response differences for salivary and plasma cortisol were detectable after administration of either low-dose or high-dose ACTH, indicating that primary adrenal insufficiency is unlikely to play a significant role in the etiology of chronic fatigue syndrome.