The comorbidity between borderline personality disorder (BPD) and antisocial personality disorder (ASPD) is well-established, and the 2 disorders share many similarities. However, there are also differences across disorders: most notably, BPD is diagnosed more frequently in women and ASPD in men. We investigated if (a) comorbidity between BPD and ASPD is attributable to 2 discrete disorders or the expression of common underlying processes, and (b) if the model of comorbidity is true across sex. Using a clinical sample of 1,400 drug users in residential substance abuse treatment, we tested 3 competing models to explore whether the comorbidity of ASPD and BPD should be represented by a single common factor, 2 correlated factors, or a bifactor structure involving a general and disorder-specific factors. Next, we tested whether our resulting model was meaningful by examining its relationship with criterion variables previously reported to be associated with BPD and ASPD. The bifactor model provided the best fit and was invariant across sex. Overall, the general factor of the bifactor model significantly accounted for a large percentage of the variance in criterion variables, whereas the BPD and AAB specific factors added little to the models. The association of the general and specific factor with all criterion variables was equal for men and women. Our results suggest common underlying vulnerability accounts for both the comorbidity between BPD and AAB (across sex), and this common vulnerability drives the association with other psychopathology and maladaptive behavior. This in turn has implications for diagnostic classification systems and treatment. General Scientific Summary This study found that, for both men and women, borderline (BPD) and antisocial personality disorders (ASPD) show a large degree of overlap, and little uniqueness. The commonality between BPD and ASPD mainly accounted for associations with criterion variables. This suggests that BPD and ASPD show a large common core that accounts for their comorbidity.