Background A general emotional vulnerability (intense, easily triggered affective reactions) is considered a core symptom in borderline personality disorder (BPD), but evidence from psychophysiological studies for this hypothesis is not very consistent. Given the high comorbidity of posttraumatic stress disorder (PTSD) in BPD patients, current comorbid PTSD might also modulate emotional reactivity. In the present study using a script-driven imagery paradigm, idiographic aversive, disorder-specific (scenes about rejection and abandonment), and standard unpleasant, neutral, and pleasant scripts were presented to investigate emotional reactivity in patients with BPD. Methods Forty nonmedicated BPD patients and 32 healthy control subjects first read and then imagined the scripts. Acoustic startle probes were presented during reading and imagery and the eye-blink responses, as well as changes in heart rate and skin conductance level were recorded. Results Imagery of disorder-specific scripts resulted in a clear potentiation of the startle responses and increased autonomic arousal in BPD patients but not in control subjects. Borderline personality disorder patients with current comorbid PTSD ( n = 26 out of 40) showed attenuated startle potentiation during aversive imagery that was not the case in BPD patients without current PTSD. This effect was most pronounced in BPD patients with severe PTSD. Conclusions Scenes about rejection and abandonment are specifically able to activate defensive response mobilization in BPD patients. These findings suggest that BPD patients are not more physiologically reactive to emotional cues in general but show increased emotional vulnerability if borderline-specific schemas are activated. Moreover, emotional reactivity is attenuated in BPD patients with PTSD.