Most biological findings in posttraumatic stress disorder (PTSD) are compatible with those of the chronic stress response, such as increased corticotropin-releasing factor (CRF) concentrations, catecholamine depletion within the central nervous system, and reduced hippocampal volume. However, over the last 10 years, biological observations have been made in PTSD that are different from what has been typically associated with chronic stress, notably certain hypothalamic-pituitary-adrenal (HPA) axis findings. In particular, urinary and plasma cortisol levels are considerably lower in PTSD patients than in non-PTSD trauma survivors and normal controls. Furthermore, the circadian pattern of cortisol release from the adrenal glands follows a greater dynamic range in PTSD than in patients with major depression or in normal controls. The reduction in cortisol levels results from an enhanced negative feedback by cortisol, which is secondary to an increased sensitivity of glucocorticoid receptors in target tissues. This HPA axis alteration contrasts with the well-known chronic stress cascade in which CRF release results in erosion of negative feedback and down-regulation of glucocorticoid receptors. Sensitization of the HPA axis is consistent with the clinical picture of hyperreactivity and hyperresponsiveness in PTSD.