OBJECTIVE: The purpose of the study was to determine if patients with a history of major depressive episode and comorbid posttraumatic stress disorder (PTSD) have a higher risk for suicide attempt and differ in other measures of suicidal behavior, compared to patients with major depressive episode but no PTSD. In addition, to explore how PTSD comorbidity might increase risk for suicidal behavior in major depressive episode, the authors investigated the relationship between PTSD, cluster B personality disorder, childhood sexual or physical abuse, and aggression/impulsivity. METHOD: The subjects were 230 patients with a lifetime history of major depressive episode; 59 also had lifetime comorbid PTSD. The demographic and clinical characteristics of subjects with and without PTSD were compared. Multivariate analysis was used to examine the relationship between suicidal behavior and lifetime history of PTSD, with adjustment for clinical factors known to be associated with suicidal behavior. RESULTS: Patients with a lifetime history of PTSD were significantly more likely to have made a suicide attempt. The groups did not differ with respect to suicidal ideation or intent, number of attempts made, or maximum lethality of attempts. The PTSD group had higher objective depression, impulsivity, and hostility scores; had a higher rate of comorbid cluster B personality disorder; and were more likely to report a childhood history of abuse. However, cluster B personality disorder was the only independent variable related to lifetime suicide attempts in a multiple regression model. CONCLUSIONS: PTSD is frequently comorbid with major depressive episode, and their co-occurrence enhances the risk for suicidal behavior. A higher rate of comorbid cluster B personality disorder appears to be a salient factor contributing to greater risk for suicidal acts in patients with a history of major depressive episode who also have PTSD, compared to those with major depressive episode alone.