Hypofunctioning of the pituitary-adrenal axis has been suggested as the pathophysiological basis for chronic fatigue syndrome (CFS). Blunted adrenocorticotropin (ACTH) responses but normal cortisol responses to exogenous corticotropin-releasing hormone (CRH), the main regulator of this axis, have been previously demonstrated in CFS patients, some of whom had a comorbid psychiatric disorder. We wished to re-examine CRH activation of this axis in CFS patients free from concurrent psychiatric illness. A sample of 14 patients with CDC-diagnosed CFS were compared with 14 healthy volunteers. ACTH and cortisol responses were measured following the administration of 100 microg ovine CRH. Basal ACTH and cortisol values did not differ between the two groups. The release of ACTH was significantly attenuated in the CFS group (P < 0.005), as was the release of cortisol (P < 0.05). The blunted response of ACTH to exogenous CRH stimulation may be due to an abnormality in CRH levels with a resultant alteration in pituitary CRH receptor sensitivity, or it may reflect a dysregulation of vasopressin or other factors involved in HPA regulation. A diminished output of neurotrophic ACTH, causing a reduced adrenocortical secretory reserve, inadequately compensated for by adrenoceptor upregulation, may explain the reduced cortisol production demonstrated in this study.