Suicidal behavior often accompanies both borderline personality disorder (BPD) and severe mood disorders, and comorbidity between the two appears to further increase suicide risk. The current study aims to quantify the risk of suicidality conferred by comorbid BPD diagnosis or features in three affective disorders: major depressive disorder (MDD), bipolar disorder (BP) and schizoaffective disorder. One hundred forty-nine (149) psychiatric inpatients were assessed by SCID I and II, and the Columbia Suicide Severity Rating Scale. Logistic regression analyses investigated the associations between previous suicide attempt and BPD diagnosis or features in patients with MDD, BP, and schizoaffective disorder, as well as a history of manic or major depressive episodes, and psychotic symptoms. Comorbid BPD diagnosis significantly increased suicide risk in the whole sample, and in those with MDD, BP, and history of depressive episode or psychotic symptoms. Each additional borderline feature also increased risk of past suicide attempt in these same groups (excepting BP) and in those with a previous manic episode. Of the BPD criteria, only unstable relationships and impulsivity independently predicted past suicide attempt. Overall, among patients with severe mood disorders, the presence of comorbid BPD features or disorder appears to substantially increase the risk of suicide attempts.