Self-directed aggression, whether in the form of non-suicidal self-mutilation or suicidal behavior, is a prominent feature of personality disorders. We hypothesized that self-injurious behavior, like suicidal behavior, represents a form of self-directed aggression, and may, like suicidal behavior and impulsive aggression, be associated with a decrease in central serotonin function in personality disorder patients. Ninety-seven patients with DSM-III personality disorder underwent D,L-fenfluramine challenge as an assessment of serotonergic activity. Patients with a history of self-mutilation or suicide had blunted prolactin and cortisol responses to D,L-fenfluramine compared to those with neither, and those with both had the most blunted responses to fenfluramine. These data raise the possibility that the central 5-HT abnormality, previously associated with suicidal behavior, may be associated with self-directed violence and not necessarily specifically with suicidal intent.