The authors compared three instruments used to diagnose borderline personality disorder--the Diagnostic Interview for Borderline Patients (DIB), the Schedule for Interviewing Borderlines, and the Structured Interview for DSM-III Personality Disorders--in 56 patients with personality disorders. A borderline diagnosis was made according to the DIB in 30%, the Structured Interview for DSM-III Personality Disorders in 48%, and the Schedule for Interviewing Borderlines in 55% of the patients. Diagnostic agreement was only 52%, which has implications for the generalizability of results of validation studies of the borderline diagnosis. Improvement in diagnostic agreement requires modification of current criteria sets and/or the use of dimensional models.