Background <p>Borderline personality disorder (BPD) is characterized by mental representation deficits and emotion dysregulation, with non-suicidal self-injury (NSSI) often occurring as a maladaptive regulation strategy. The ability to verbally express emotions might be advantageous for coping with emotion dysregulation and benefiting from psychotherapy.</p> Methods <p>In the present study, we used a novel text-based measure of emotional awareness to examine whether a greater emotion word repertoire (EWR) predicts improvement in psychotherapy for patients with BPD regarding NSSI, suicide attempts, attachment representations, mentalization, and personality organization. We conducted a secondary analysis of a randomized controlled trial comparing the efficacy of Transference-Focused Psychotherapy (TFP) vs. treatment as usual over one year in a sample of female BPD outpatients. The German electronic Levels of Emotional Awareness Scale (eLEAS) scoring system was applied to Adult Attachment Interviews (AAI) administered at baseline (<i>n</i> = 87; <i>M</i>_age = 27.4, <i>SD</i>_age = 7.4) and upon treatment termination (<i>n</i> = 52; <i>M</i>_age = 28.6, <i>SD</i>_age = 7.2).</p> Results <p>In both treatment groups, EWR at baseline was positively correlated with a reduction of NSSI after one year of psychotherapy (<i>r</i> = .46, <i>p</i> &lt; .001). No significant correlations were found between baseline EWR and changes in other outcome measures. Compared to baseline, mean EWR scores significantly decreased after one year of treatment.</p> Discussion <p>Our findings indicate that a borderline patient’s ability to verbalize emotions might be a resource facilitating a reduction of NSSI in psychotherapy. We discuss strengths and limitations of applying the eLEAS scoring system to open-ended texts in a psychotherapy context. Given the exploratory nature of this study, replication in future studies is warranted.</p> Trial registration <p>ClinicalTrials.gov (identifier NCT00714311, registration date 07/09/2008).</p>