This study had two aims: to test the hypothesis that borderline personality disorder (BPD) patients hold numerous dysfunctional beliefs associated with a variety of Axis II disorders, and to construct a BPD belief scale which captures these beliefs. Beliefs were measured using the Personality Belief Questionnaire (PBQ) which is designed to assess beliefs associated with various personality disorders, although not specifically BPD. Eighty-four BPD patients and 204 patients with other personality disorders (OPD) were randomly split into two study samples. Fourteen PBQ items were found to discriminate BPD from OPD patients in both samples. These items came from the PBQ Dependent, Paranoid, Avoidant, and Histrionic scales and reflect themes of dependency, helplessness, distrust, fears of rejection/abandonment/losing emotional control, and extreme attention-seeking behavior. A BPD beliefs scale constructed from these items showed good internal consistency and diagnostic validity among the 288 study patients. The scale may be used to assist in diagnosis and cognitive therapy of BPD.