The notion that chronic stress fosters disease by activating the hypothalamic-pituitary adrenocortical (HPA) axis is featured prominently in many theories. The research linking chronic stress and HPA function is contradictory, however, with some studies reporting increased activation, and others reporting the opposite. This meta-analysis showed that much of the variability is attributable to stressor and person features. Timing is an especially critical element, as hormonal activity is elevated at stressor onset but reduces as time passes. Stressors that threaten physical integrity, involve trauma, and are uncontrollable elicit a high, flat diurnal profile of cortisol secretion. Finally, HPA activity is shaped by a person's response to the situation; it increases with subjective distress but is lower in persons with posttraumatic stress disorder.