Interpersonal dysfunction is a core characteristic of borderline personality disorder (BPD). Cross-sectional research suggests that higher BPD features are associated with lower relationship quality. Many researchers have examined the associations between interpersonal stressors and short-term increases in emotional reactivity or impulsivity among those with high levels of BPD features. Yet, most research paradigms assume that interpersonal stressors change over time, whereas BPD features remain relatively stable; however, the stability of BPD features in the context of fluctuating interpersonal stressors has yet to be tested longitudinally. In the current study, we examined co-occurring changes in relationship quality and BPD features over time. In Study 1, 126 female, undergraduate participants completed ratings of relationship quality (i.e., conflict, criticism, closeness, satisfaction, and support) with recent interpersonal partners as well as self-reported BPD features at baseline and 1-month follow-up. In Study 2, 50 female participants from the community (n = 27 with BPD; n = 23 without any psychiatric diagnoses) completed the same measures at baseline and 1-, 3-, and 6-month follow-ups. Across both studies, when participants reported decreases in the quality of their interpersonal relationships, they endorsed increases in BPD features. Similarly, when participants endorsed worsening BPD features, their perceptions of relationship quality with all recent interaction partners (in Study 1) or more frequent interaction partners (in Study 2) decreased as well. Even in samples with a wide range of BPD features, these results suggest that the quality of interpersonal relationships and BPD features may influence each other over time. (PsycInfo Database Record (c) 2022 APA, all rights reserved).