To examine the prolactin (prl) response to d,l-fenfluramine in a large sample of personality disorder patients with impulsive aggression. Patients were screened from clinics at the Bronx VAMC and the Mount Sinai Medical Center and from press releases. One hundred and forty-six personality disorder patients (90M;56F) and 23 normal controls (15M;8F) underwent oral d,l-fenfluramine challenge. The peak change in prolactin(deltapkprl) was calculated by subtracting baseline prolactin from peak response following fenfluramine administration (3 h). Analysis of variance and regression analysis were used to detect group differences in deltapkprl. Deltapkprl in impulsive aggressive men, but not women, with personality disorders was blunted compared with controls. Men with suicide histories also had a blunted deltapkprl compared with those without, which was not accounted for by depression. This study represents a replication of previous studies, in a much larger sample, showing a blunted PRL response to fenfluramine of male patients with personality disorder in relation to impulsive aggression and to suicide attempts.