Evidence-based treatments for borderline personality disorder (BPD), including mentalization-based treatment (MBT), have not adequately examined changes in positive affect (PA) in the treatment of BPD. Therefore, we developed a new intervention, "mentalizing positive affect," and evaluated its effect on PA, negative affect, BPD severity, ego-resiliency, and quality of life during MBT treatment for BPD. In a single-case multiple-baseline design, 4 female BPD patients received 6 months of individual MBT, after which they were followed up for 2 months. Intensive repeated measurements data were subjected to hierarchical linear modeling to analyze whether the positive intervention was related to changes in self-reported outcome measures. Our results failed to support a co-occurring increase in the reporting of PA related to the "mentalizing positive affect" intervention. However, the slope of PA increased at a quicker rate after the end of treatment, perhaps indicating a delayed treatment effect. "Mentalizing positive affect" was related to a marginally significant decrease in the mean level of BPD severity compared with standard MBT. Moreover, focusing on PA in MBT seemed feasible for maintaining a good working alliance. Our findings call for more research to test interventions aimed at enhancing PA in the treatment of BPD. Such efforts might well involve treatment of longer duration and higher intensity to increase the number of sessions, as well as longer follow-up periods, than we used. (PsycInfo Database Record (c) 2020 APA, all rights reserved).