The authors describe the content, administration, reliability, construct validity, and training requirements for a semistructured diagnostic interview for borderline patients. The interview is composed of operationally defined and scored variables that assess five areas of functioning considered most characteristic of borderline patients--social adaptation, impulse/action patterns, affects, psychosis, and interpersonal relations. Their results show that the interview provides a reasonably constructed and reliable method for researchers and clinicians interested in identifying the syndrome of borderline personality disorders. Although existing research supports the clinical utility of defining borderline patients with the diagnostic interview for borderline patients, both the diagnosis and the content of the interview may require revisions.