This study assessed the instability of romantic partner trustworthiness perceptions as a possible underlying mechanism of unstable relationships in borderline personality disorder (BPD). Based on recent empirical findings of difficulties in maintaining trust in economic games and negatively biased facial trust perceptions in BPD individuals, the authors hypothesized that trustworthiness attributions with regard to a romantic partner could also be unstable and negatively biased. To assess this, the authors recruited a sample of 36 female BPD patients and their male partners and compared them to 36 heterosexual couples of healthy control participants that were matched for age, intelligence and relationship length. Couples were instructed to discuss three topics with each other, after which they provided trustworthiness ratings of their partner. First, couples discussed a neutral topic (favourite film), followed by a personally threatening topic (e.g. negative event in past year) and finally a relationship threatening topic (possible reasons for separation from their partner). The authors hypothesized that BPD individuals would show unstable attributions of their partners' trustworthiness depending on the context, judging them as less trustworthy after threatening conversations. Indeed, women with BPD judged their partners as significantly less trustworthy than control participants after both threatening discussions but not after the neutral discussion. The authors conclude that the context dependency of attributed partner trustworthiness may contribute to instability in romantic relationships of those with BPD. In addition to the effect of the situation on trustworthiness ratings, the authors assessed potential protective factors that are associated with higher stability in trustworthiness ratings and identified relationship tenderness as such a factor. In light of most previous studies assessing interpersonal problems or relationship quality in BPD individuals, but not directly the instability of such relationships, this study is a first step to a more direct test of the mechanisms underlying the DSM-5 criterion of unstable relationships in BPD and opens the door for the future assessment of factors beyond trustworthiness perceptions.