Introduction: Borderline Personality Disorder (BPD) is defined by marked instability in the areas of identity, personal relationships, mood, and affect, as well as impulsive behavior, periods of intense anger and rage, feelings of emptiness, suicidal behavior, self-harm, transient, stress-related paranoid ideas, and pronounced dissociative symptoms (Leichsenring et al., 2023). The prevalence of BPD in the general population is estimated to range from 1.4% to 5.9% (Aragonès et al., 2013), while it is significantly higher in clinical settings. Specifically, it is estimated that approximately 10% to 12% of patients in outpatient psychiatric treatment have BPD, and 20% to 22% of patients in inpatient settings (Ellison et al., 2018). Over the past thirty years, research in the field of the etiology of BPD has particularly focused on the role of early adverse experiences. Studies consistently point to the importance of early adverse experiences and trauma in understanding the etiology of BPD (De Aquino Ferreira et al., 2018; Widom et al., 2009; Zanarini et al., 1997). According to the biopsychosocial model (Linehan, 1993), BPD primarily arises as a result of an emotional regulation disorder that develops in the context of the interaction between biological vulnerability and specific environmental influences. Difficulties in emotional regulation refer to a lack of awareness and clarity about emotional responses, non-acceptance of these responses, limited access to effective emotional regulation strategies, difficulties in directing behavior toward goals in situations of high emotional arousal, and impulse control difficulties (Gratz & Roemer, 2004). Cognitiveemotional regulation strategies, on the other hand, refer to ways of regulating and managing emotions through the use of various strategies primarily based on cognitive processes. Research clearly indicates that individuals with symptoms of BPD more frequently use maladaptive and dysfunctional strategies (e.g., rumination and catastrophizing) and less frequently adaptive strategies (e.g., positive refocusing and planning), and that the use of dysfunctional strategies is also associated with greater difficulties in general functioning (Akyunus et al., 2020; Daros et al., 2018; Daros & Ruocco, 2021). In addition to early adverse experiences and difficulties in emotional regulation, mentalization also plays an important role in understanding the etiology and symptoms of BPD. Fonagy and Bateman (2008, p. 5) define mentalization as “the capacity to implicitly and explicitly understand ourselves and others in terms of subjective states and mental processes.” Fonagy and Bateman (2008) propose a model in which inadequate parental mirroring of affect and a disorganized attachment pattern in the context of trauma predispose the development of BPD through impairments in mentalization. Previous research has repeatedly confirmed the connection between mentalization and BPD, indicating that difficulties in understanding complex models of interpersonal interactions, confusion and uncertainty about one's own and others' internal states (i.e., hypomentalization) are particularly important (Duval et al., 2018; Euler et al., 2021; Fonagy et al., 2016). The relationship between mentalization and symptoms of PTSD, depression, anxiety, and self-harm has also been confirmed (Fonagy et al., 2016; Kennedy-Turner et al., 2022; Martin Gagnon et al., 2023; WagnerSkacel et al., 2022). In a series of studies by Fonagy and colleagues (2016), it has been shown that lower mentalization is significantly associated with lower quality of life, psychosocial problems, poorer anger control, weaker reality testing, diffuse identity, internalized and externalized problems, and narcissism. This review of research highlights the importance of understanding the relationship between early adverse experiences, mentalization, and cognitive-emotional regulation strategies in explaining the symptoms of BPD. It is evident that previous research points to the significant role of early adverse experiences, which are associated with a reduced capacity for mentalization and effective emotional regulation, thereby increasing the risk of developing symptoms of BPD, characterized by pervasive and persistent instability in mood, self-image, and relationships with others. Research aim and problems: The aim of the research was to examine the role of early adverse experiences, mentalization, and cognitive-emotional regulation strategies in explaining the severity of Borderline Personality Disorder (BPD) symptoms in adulthood. The first problem was to investigate the role of early adverse experiences, mentalization, and adaptive and maladaptive cognitive-emotional regulation strategies in explaining BPD symptoms in adulthood. The second problem was to examine the mediating role of maladaptive cognitive-emotional regulation strategies in the relationship between mentalization and BPD features in adulthood. According to the first hypothesis, it was expected that a higher number of early adverse experiences and lower mentalization would be positive predictors of BPD symptoms in adulthood. According to the second hypothesis, it was expected that adaptive cognitive-emotional regulation strategies would be negative predictors of BPD traits in adulthood, while maladaptive cognitive-emotional regulation strategies would be positive predictors. Finally, according to the third hypothesis, it was expected that maladaptive cognitive-emotional regulation strategies (rumination, catastrophizing, self-blame, and blaming others) would be partial mediators of the relationship between mentalization and BPD traits in adulthood. Methods: A total of 486 participants were included, of which 330 were female (67.9%), ranging in age from 18 to 73 years, with an average age of M = 36.1 (SD = 11.3). The research was conducted online in December 2023 using a questionnaire distributed via the SurveyMonkey platform and shared on social networks. Data were also collected using the snowball sampling, where participants were asked to forward the link to their groups, acquaintances, and friends, who were then also asked to forward it further. The Sociodemographic Questionnaire, Borderline Symptom List 23 (BSL-23; Bohus et al., 2009), Reflective Functioning Questionnaire (RFQ-8; Fonagy et al., 2016), Cognitive Emotion Regulation Questionnaire—Short Form (CERQ-short; Garnefski & Kraaij, 2006), and Revised Inventory of Adverse Childhood Experiences (Finkelhor et al., 2015) were used. Results: To examine the relationship between BPD symptoms, early adverse experiences, mentalization, cognitive-emotional regulation strategies and sociodemographic variables (age, gender, and education level), correlation coefficients were calculated. The obtained correlation coefficients indicate that individuals with more pronounced BPD symptoms have more early adverse experiences, are less confident in assessing their own and others' mental states, more frequently use catastrophizing, rumination, self-blame, and blaming others, and less frequently use positive reappraisal, refocusing on the positive, planning, and putting into perspective. To determine how much variance in BPD symptoms can be explained by early adverse experiences, mentalization, and maladaptive and adaptive cognitive-emotional regulation strategies, a hierarchical regression analysis was conducted. The overall model explained 46% of the variance in the criteria, with standardized beta coefficients indicating that early adverse experiences were the strongest positive predictor of BPD symptoms in the final model, followed by mentalization, catastrophizing, rumination, and self-blame. Blaming others and adaptive cognitive-emotional regulation strategies (acceptance, planning, positive reappraisal, refocusing on the positive, and putting into perspective) were not statistically significant predictors. Finally, mediation analysis showed that mentalization affects BPD symptoms both directly and indirectly, such that individuals with greater difficulties in mentalization more frequently use maladaptive cognitive-emotional regulation strategies, which are then associated with more pronounced features of BPD. Conclusion: The research indicates that a higher number of early adverse experiences, mentalization, and maladaptive cognitive-emotional regulation strategies (catastrophizing, rumination, and self blame) are significant for understanding BPD symptoms. It also showed that mentalization affects BPD symptoms both directly and indirectly, such that individuals with greater difficulties in mentalization more frequently use maladaptive cognitive-emotional regulation strategies, which are then associated with more pronounced features of BPD. The results obtained have both scientific and practical implications. The findings suggest that working with individuals diagnosed with BPD should focus on raising awareness and strengthening the function of mentalization, gaining confidence in the ability to control internal states and their differences from reality, which in itself strengthens the capacity for emotional regulation. Additionally, the results indicate the importance of working on raising awareness and restructuring maladaptive cognitive-emotional regulation strategies.