OBJECTIVE:According to mentalization-based theory, transgenerational transmission of mentalization from caregiver to offspring is implicated in the pathogenesis of borderline personality disorder (BPD). Recent research has demonstrated an association between hypermentalizing (excessive, inaccurate mental state reasoning) and BPD, indicating the particular relevance of this form of mentalizing dysfunction to the transgenerational mentalization-based model. As yet, no study has empirically assessed a transgenerational mentalization-based model of BPD. The current study sought firstly to test the mentalization-based model, and additionally, to determine the form of mentalizing dysfunction in caregivers (e.g., hypo- or hypermentalizing) most relevant to a hypermentalizing model of BPD. METHOD:Participants were a mixed sample of adolescents with BPD and a sample of non-clinical adolescents, and their respective primary caregivers (n = 102; 51 dyads). Using an ecologically valid measure of mentalization, mediational analyses were conducted to examine the relationships between caregiver mentalizing, adolescent mentalizing, and adolescent borderline features. RESULTS:Findings demonstrated that adolescent mentalization mediated the effect of caregiver mentalization on adolescent borderline personality pathology. Furthermore, results indicated that hypomentalizing in caregivers was related to adolescent borderline personality pathology via an effect on adolescent hypermentalizing. CONCLUSIONS:Results provide empirical support for the mentalization-based model of BPD, and suggest the indirect influence of caregiver mentalization on adolescent borderline psychopathology. Results further indicate the relevance of caregiver hypomentalizing to a hypermentalizing model of BPD.