Interpersonal dysfunction is considered a cornerstone of borderline personality disorder (BPD). Relationships are described as intense and unstable, with individuals with BPD alternating between idealization and devaluation of relationship partners. Furthermore, a lack of stable and supportive relationships may be related to symptom maintenance and exacerbation. Despite widespread recognition of the importance of relationship instability in BPD, there is little empirical evidence about the nature of such instability and how it emerges over time. We examined the stability of social networks of women diagnosed with BPD (n = 27) and healthy controls (HCs; n = 23) by assessing key characteristics of relationships (satisfaction, support, closeness, conflict, and criticism) over a 6-month period. We conducted analyses to examine whether relationship instability depended on the frequency of interaction with members of the network. Results showed that the relationships of women in the BPD group were perceived as more unstable than those of the HC group. Compared with women in the HC group, women with BPD had networks with more relationships that had undergone significant change or had been "cut off" over the course of the study. The relationship between frequency of interaction and instability in support and satisfaction differed between groups. Women in the HC group showed greater instability in support with partners they interacted with infrequently, whereas women in the BPD group showed greater instability in satisfaction with partners they interacted with more frequently. Implications for understanding interpersonal dysfunction in BPD are discussed and possible areas of relevance for treatment development are highlighted. (PsycInfo Database Record (c) 2020 APA, all rights reserved).