The Diagnostic and Statistical Manual of Mental Disorders (DSM)-5 Personality and Personality Disorders Work Group proposed the elimination of diagnostic criterion sets in favor of a prototype matching system that defines personality disorders using narrative descriptions. Although some research supports this general approach, no empirical studies have yet examined the specific definitions proposed for DSM-5. Given the wide interest in borderline personality disorder (BPD), it is crucial to determine how this methodological shift might affect the content and conceptualization of the diagnosis. Eighty-two experts on BPD provided ratings of the DSM-IV-TR or DSM-5 version of BPD in terms of 37 traits proposed for DSM-5. Analyses revealed significant and meaningful differences among the two constructs, with the DSM-5 version evincing increased interpersonal dependency but a decreased emphasis on antagonism and disinhibition. A second study within a clinical sample demonstrated that both antagonism and disinhibition mediated the relationships between DSM-IV BPD and impairment, suggesting that the proposed changes might have important consequences for BPD's coverage, prevalence, and nomological network. More globally, our results illustrate that unanticipated shifts in diagnostic constructs can stem from seemingly minor revisions and suggest that research is needed to understand how these, or other changes, might affect our conceptualization, diagnosis, and treatment of BPD.