Interview methods are widely regarded as the standard for the diagnosis of borderline personality disorder (BPD), whereas self-report methods are considered a time-efficient alternative. However, the relative validity of these methods has not been sufficiently tested. The current study used data from the Collaborative Longitudinal Personality disorder Study to compare diagnostic base rates and the relative validity of interview and self-report methods for assessing functional outcome in BPD. Although self-report yielded higher base rates of criteria endorsement, results did not support the common assumption that diagnostic interviews are more valid than self-reports, but instead indicated the combined use of these methods optimally identifies BPD criteria.