Chronic fatigue syndrome (CFS) is a disease characterized by debilitating fatigue of at least 6 months' duration. The pathophysiology and the localization of the underlying HPA axis disturbance are a matter of debate. Our aim was to evaluate the hypothalamopituitary-adrenal (HPA) axis by the 1 mu g adrenocorticotrophic hormone (ACTH) test and metyrapone test in patients with CFS and to compare the size of the adrenal glands of the patients with that of the control subjects. Twenty patients (14 females, 6 males) with CFS were included in the study. Fifteen healthy subjects (12 females, 3 males) served as matched controls. ACTH stimulation test was carried out by using 1 mu g ACTH, intravenously, as a bolus injection after an overnight fast, and blood samples for cortisol were drawn at 0, 30, and 60 minutes. Metyrapone at a dosage of 30 mg/kg was taken orally at 11:00 PM with a snack. The following morning, blood was sampled for serum 11-deoxycortisol between 8:00 and 9:00 AM. Peak cortisol responses to 1 mu g ACTH test were significantly lower in the CFS group (620.7 +/- 146.2 nmol/L) than in the control group (838.7 +/- 129.6 nmol/L) (P < 0.05). The lowest peak cortisol response after the 1 mu g ACTH test was 575.8 nmol/L in normal subjects. Nine patients with CFS had peak cortisol responses to 1 mu g ACTH test lower than 575.8 nmol/L. 11-deoxycortisol response to metyrapone was significantly lower in the patients with CFS (114.2 +/- 31.1 nmol/L) than in the healthy subjects (186.5 +/- 15.7 nmol/L) (P < 0.05). The lowest 11-deoxycortsol level after metyrapone in the controls was 168.3 nmol/L. Nineteen of 20 patients with CFS had lower 11-deoxycortisol level after metyrapone than detected in normal subjects. Eight patients had both subnormal 11-deoxycortisol response to metyrapone and subnormal cortisol response to 1 mu g ACTH test. On the other hand, 10 patients had subnormal 11-deoxycortisol response to metyrapone but normal cortisol response to 1 mu g ACTH test. Thickness of the right and left adrenal glands were similar between the patients with CFS and healthy subjects (P > 0.05). We conclude that the perturbation of the HPA axis in CFS is characterized by underactivation of the HPAaxis.