OBJECTIVES: Among mood disorders, bipolar disorder (BPD) is often noted to involve the highest rates of suicide attempts and possibly of completion. This study sought to determine whether suicide attempters with BPD exhibit suicide attempts with higher lethality than attempters with major depressive disorder (MDD) and to explore differences in clinical features associated with suicidal acts. METHODS: Mood disordered suicide attempters were interviewed about Axis I and II diagnoses, lifetime history of suicide attempts, suicidal intent, suicidal ideation, the medical lethality of their most severe suicide attempt, severity of depression, hopelessness, lifetime aggression, and impulsivity. RESULTS: The maximum lethality of suicidal acts tended to be higher among BPD attempters compared with those with MDD. However, there were no differences in the number of suicide attempts, intent to die or suicidal ideation. Suicide attempters with BPD reported higher levels of aggression and impulsivity but less hopelessness compared with MDD attempters. These differences could not be explained by Cluster B personality disorder comorbidity. Of note, within the BPD group, but not the MDD group, males reported suicidal acts with higher lethality. Multivariate analyses suggested that risk for more lethal suicide attempts is associated with BPD and male sex and that bipolar males appear to be especially vulnerable to these behaviors. CONCLUSIONS: Males with BPD make more lethal suicide attempts than females with BPD, an effect not observed among the MDD sample. Our findings suggest that higher rates of suicidal behavior in BPD may be due to a specific effect of BPD on males, leading to more dangerous suicidal behaviors. This effect, together with the larger proportion of males in the BPD group compared with the MDD group may lead to higher rates of reported attempted and completed suicide.