No lay-administered interviews are currently available to identify persons with borderline personality disorder. The authors studied 79 subjects with the NIMH Diagnostic Interview Schedule (DIS), a lay-administered interview, and the Diagnostic Interview for Borderline Patients (DIB) and used the results to construct a DIS-based diagnostic index to identify borderline personality disorder. Using the clinician-administered DIB as the diagnostic standard, the authors found that the DIS borderline index had a sensitivity of 85.7%, a specificity of 86.2%, and a kappa of 0.67. The DIS borderline index is a promising extension of the DIS that will facilitate studies of borderline personality disorder in clinical and community settings.