Defense reactions to threatening situations are vital adaptations to stress that protect organisms from injury and ensure survival. We retrospectively investigated the role of peritraumatic dissociation (PD) in the occurrence of severe psychopathology and dissociative patterns of reactions in borderline personality disorder (BPD). We recruited 28 patients with a clinical diagnosis of BPD and 15 healthy controls. The BPD group was divided according to the level of PD (low vs. high): BPD and PD (n = 15) and BPD only (n = 13). We conducted an extensive investigation of history of trauma, clinical status, and measurements of emotional and physiologic responses to recall of personalized aversive experiences. Participants with BPD and high PD displayed highest degrees of trauma exposure and clinical symptoms. Their significant heart rate decline during the imagery of personal traumatic events was opposed to the heart rate increases exhibited by the other two groups and may indicate a dissociative reaction pattern. Skin conductance responses did not differentiate between groups. Several emotional responses to imagery also reinforced the idea that PD may play a role in memory processing of traumatic events and thus in the aggravation and maintenance of symptoms in particularly severe forms of BPD. Within a stepwise linear regression analysis, the best model for trauma‐evoked heart rate responses included PD and borderline symptoms, but no measures of state or trait dissociation. Our findings may provide initial evidence of an evolutionary model of peritraumatic reaction stages evolving from arousal to dissociation.