Sixteen female outpatients with borderline personality disorder and prominent behavioral dyscontrol, but without a current episode of major depression, were studied in a double-blind, crossover trial of placebo and the following four active medications: alprazolam (average dose, 4.7 mg/d); carbamazepine (average dose, 820 mg/d); trifluoperazine hydrochloride (average dose, 7.8 mg/d); and tranylcypromine sulfate (average dose, 40 mg/d). Each trial was designed to last six weeks. Tranylcypromine and carbamazepine trials had the highest completion rates. Physicians rated patients as significantly improved relative to placebo while receiving tranylcypromine and carbamazepine. Patients rated themselves as significantly improved relative to placebo only while receiving tranylcypromine. Patients who tolerated a full trial of trifluoperazine showed improvement, those receiving carbamazepine demonstrated a marked decrease in the severity of behavioral dyscontrol, and those receiving alprazolam had an increase in the severity of the episodes of serious dyscontrol. As an adjunct to psychotherapy, pharmacotherapy can produce modest but clinically important improvement in the mood and behavior of patients with borderline personality disorder. As a research tool, patterns of pharmacological response may provide clues to biological mechanisms underlying dysphoria and behavioral dyscontrol.