Linehan's biosocial theory of borderline personality disorder (BPD) was tested in a sample of 1,044 female college students. Relationships between self-reported BPD symptoms, two personality traits (negative emotionality and constraint), and three key variables from the biosocial theory (emotional vulnerability, invalidation, and emotional dysregulation) were examined using structural equation modeling. Consistent with the biosocial theory, the best fitting model Indicated that emotional vulnerability and emotional dysregulation were uniquely related to BPD symptoms after controlling for personality traits, and that dysregulation mediated the relationship between emotional vulnerability and BPD. However, invalidation was not significantly associated with BPD and could be dropped from that model. The full model with Linehan's constructs and personality traits explained 58% of the variance in BPD and, as expected, explained considerably less variance in symptoms of other Cluster B PDs. The present results highlight the incremental value of the Linehan's theory and its specificity to BPD.