OBJECTIVES Training and practice in medicine are inherently stressful. Research into the effects of acute stressors has revealed significant variability in individual responses to stressors, with performance impairments occurring in those who demonstrate elevated subjective and physiological responses. Cognitive appraisals (subjective assessment of situational demands and available resources) of a stressor have been proposed as a predictor variable in stress responses. However, the relationship between cognitive appraisal and stress responses has not been tested empirically in complex realistic situations. The purpose of this study was to determine the extent to which cognitive appraisal affects a medical trainee's subjective and physiological stress responses to high-acuity simulated clinical situations. METHODS Thirteen emergency medicine and general surgery residents participated in high (HS) and low (LS) stress trauma resuscitation simulations. Subjective (cognitive appraisal and State-Trait Anxiety Inventory [STAI]) and physiological (salivary cortisol) measures were collected at baseline and in response to participation in each scenario. RESULTS Post-scenario STAI scores, cognitive appraisal and cortisol levels were higher in the HS scenario compared with the LS scenario. For the participants who appraised the scenarios as 'threats' (in which the demands outweighed the resources), the ratio of perceived demands to resources was positively correlated with cortisol levels (r = 0.59, p < 0.05) and STAI responses (r = 0.64, p < 0.05). By contrast, for the participants who appraised the scenarios as 'challenges' (in which resources were sufficient to meet the demands), the perceived ratio of demands to resources was not correlated with either the STAI scores or cortisol levels. CONCLUSIONS Subjective appraisals of a situation appear to play an important role in stress responses, which have previously been shown to impair performance. As such, training for high-acuity events should include interventions targeting stress management skills.