Objective: Chronic fatigue immune dysfunction syndrome (CFIDS) is a debilitating disease characterized by physical and mental fatigue for at least 6 months. The etiology of CFIDS is heterogeneous. Hypothalamic-pituitary-adrenal (HPA) axis dysfunction is considered one of the underlying pathophysiological mechanisms. The results of the studies investigating HPA axis activity in CFIDS are quite conflicting. This study aimed to investigate HPA axis activity in patients with CFIDS and the effect of corticosteroid treatment on HPA axis activity. Method: Thirty-four patients with CFIDS who did not have any psychiatric co-morbidity and who were diagnosed according to Centers for Disease Control (CDC) criteria and 22 physically and mentally-healthy subjects were included in the study. The HPA axis activity was evaluated by basal serum cortisol and dehydroepiandrosterone sulfate (DHEAS) and cortisol and DHEAS responses to low-dose adrenocorticotrophic hormone (ACTH) test. The procedure was repeated after four-week dexamethasone treatment (1.25mg/day). 20 patients were able to complete the trial. The procedure was performed only once in the control subjects. Results: There were no significant differences in basal serum cortisol and DHEAS levels, peak hormone responses or area under the hormone response curve between the patients and the controls in pre-treatment. While cortisol response to ACTH was statistically significant in the patients and controls, DHEAS response was not. The hormone responses to ACTH did not differ between the patients and controls. In the patients, peak DHEAS value was significantly reduced after dexamethasone treatment compared to those in pre-treatment. Conclusion: The results of the study suggest that serum cortisol and DHEAS levels and cortisol and DHEAS responses to ACTH in patients with CFIDS did not differ from healthy controls. These results do not support the hypotheses that patients with CFIDS have HPA dysfunction. Nevertheless, further studies are needed to confirm the results.