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Background: Sensitivity to punishment (SP) and sensitivity to reward (SR) are personality characteristics that may have relevance for the pathophysiology of eating disorders (EDs). Moreover, personality characteristics are known to modulate the activity of the hypothalamus-pituitary-adrenal (HPA) axis, which is the main component of the endogenous stress response system. As stress has been implicated in the aetiology and the maintenance of EDs, we aimed to study the HPA axis activity in relation to SP and SR, as conceptualized by Gray's reinforcement sensitivity theory (RST), in patients with anorexia nervosa (AN) or bulimia nervosa (BN)., Method: Twenty-five women with AN, 23 women with BN and 19 healthy women volunteered for the study. HPA axis activity was assessed by measurement of the salivary cortisol awakening response (CAR). The subjects' SP and SR were measured by the behavioural inhibition system (BIS)/behavioural approach system (BAS) scales., Results: The CAR was significantly enhanced in AN patients, but not in BN patients, compared to healthy women. The CAR correlated significantly with BAS measures, negatively in healthy controls and positively in binge-purging AN patients and BN women. SP, measured by the BIS scale, was higher in patients than in controls., Conclusions: These findings confirm the occurrence of an enhanced activity of the HPA axis in symptomatic AN, but not in symptomatic BN, and show for the first time that the CAR is associated with SR, as conceptualized by the RST, negatively in healthy subjects but positively in binge-purging ED patients.