Patients with borderline personality disorder (BPD) display severe difficulties in interpersonal relationships and impulse control. We explored the possibility that patients with BPD show less trust and more risk-taking behavior in experimental games as compared with controls and with depressed patients with other personality disorders. In the trust game, the participant played the role of an investor who interacted with a trustee via the Internet. The investor could choose a costly action by giving money units (MU) to the trustee. The trustee then could honor the investor's trust by sharing the monetary increase. In the risk game, the investor could transfer money to a lottery, and therefore the payoff depended on luck and not on the decision of another person. Results revealed that the patients with BPD (n = 25) transferred a smaller amount of MUs across 5 consecutive transactions in the trust game as compared with the controls (n = 25) and with the depressed patients (n = 25). In the risk game, the performance of the BPD patients was similar to that of the controls and depressed patients. Trust game performance was predicted by the interpersonal and cognitive sector scores of the Zanarini Rating Scale for Borderline Personality Disorder. Self reports indicated that the patients with BPD were less optimistic regarding the outcome (payoff) of the trust game, but not of the risk game. These results suggest that patients with BPD exhibit less trust during interpersonal interactions, which may be related to stress-related paranoia, dissociation, identity disturbance, and problems in interpersonal relationships.