Dysfunction of the central serotonergic system has been variously associated with depression and with suicidal and/or impulsive aggressive behavior. To evaluate central serotonergic function in relation to these variables, prolactin responses to a single-dose challenge with fenfluramine hydrochloride (60 mg orally), a serotonin releasing/uptake-inhibiting agent, were examined in 45 male patients with clearly defined major affective (n = 25) and/or personality disorder (n = 20) and in 18 normal male control patients. Prolactin responses to fenfluramine among all patients were reduced compared with responses of controls. Reduced prolactin responses to fenfluramine were correlated with history of suicide attempt in all patients but with clinician and self-reported ratings of impulsive aggression in patients with personality disorder only; there was no correlation with depression. These results suggest that reduced central serotonergic function is present in a subgroup of patients with major affective and/or personality disorder and is associated with history of suicide attempt in patients with either disorder, but with impulsive aggression in patients with personality disorder only.