Objective: To determine whether borderline personality disorder (BPD) and bipolar II disorder can be differentiated from each other and from major depressive disorder (MDD) by comparing depression severity, impulsiveness, and hostility in mood disorder patients with and without BPD. Method: One hundred seventy-three patients with either MDD or bipolar II disorder were enrolled from a larger sample admitted to a multisite project on mood disorders and suicidal behavior conducted from June 1996 through June 2006. Patients were divided into 4 groups: MDD with BPD, MDD without an Axis II diagnosis, bipolar II disorder with BPD, and bipolar II disorder without an Axis II diagnosis. All diagnoses were based on DSM-IV criteria. Depression was assessed using the 17-item Hamilton Rating Scale for Depression (HAM-D) and the self-rated Beck Depression Inventory (BDI). Impulsiveness was assessed using the Barratt Impulsiveness Scale, and hostility was assessed using the Buss-Durkee Hostility Inventory. Results: Patients with BPD reported higher levels of impulsiveness (p = .004) and hostility (p = .001), independent of Axis I diagnosis. Bipolar II patients reported greater attentional impulsiveness (p = .008) than MDD patients, independent of BPD status, while BPD patients reported greater nonplanning impulsiveness than patients without BPD, independent of Axis I diagnosis (p = .02). For motor impulsiveness, there was a main effect for Axis I diagnosis (p = .05) and Axis II diagnosis (p = .002). The bipolar II + BPD group scored the highest, suggesting a compound effect of comorbidity. There were no differences in depression severity when measured with the HAM-D, although the BPD groups reported more severe depression on the BDI, independent of their Axis I diagnosis (p = .05). The BPD groups scored higher on the cognitive factor (p = .01) and anxiety factor (p = .03) of the HAM-D. Conclusion: Results suggest that there is a unique symptom and trait profile associated with BPD that distinguishes the diagnosis from bipolar II disorder. Results also suggest that impulsiveness is an important aspect of both disorders and that there is a compounding effect associated with a diagnosis of bipolar II disorder with comorbid BPD.