Previous research has examined the relationship between Borderline Personality Disorder (BPD) and other forms of psychopathology.  Although the association between Major Depressive Disorder (MDD) and BPD has received empirical (e.g., Pinto, Grapentine, Francis, & Picariello, 1996) and conceptual (e.g., DSM-IV-TR, APA, 2000) support this association falls short of fully depicting the complex etiology of BPD.  Previous research suggests that Post-traumatic Stress Disorder (PTSD) may be a cognitive-affective mediator between MDD and BPD.  Participants (n = 324) were administered the Coolidge Axis II Inventory (CATI), a measure of Axis I and II psychopathology. Multiple regression analysis confirmed the original mediating hypothesis.  These findings provide impetus for the development of a more informative model concerning the relationship between MDD and BPD.