The baseline inter-rater reliability, test-retest reliability, follow-up inter-rater reliability, and follow-up longitudinal reliability (interrater reliability between generations of raters) of borderline symptoms and the diagnosis of borderline personality disorder (BPD) were assessed using the Revised Diagnostic Interview for Borderlines (DIB-R). Excellent kappa s (> .75) were found in each of these reliability substudies for the diagnosis of BPD itself. Excellent kappa s were also found in each of the three inter-rater reliability substudies for the vast majority of borderline symptoms assessed by the DIB-R. Test-retest reliability for these symptoms was somewhat lower but still very good. More specifically, one-third of the BPD symptoms assessed had a kappa in the excellent range and the remaining two-thirds had a kappa in the fair-good range (.57-.73). The dimensional reliability of BPD symptom areas was somewhat higher than for categorical measures of the subsyndromal phenomenology of BPD. More specifically, all five dimensional measures of borderline psychopathology had intraclass correlation coefficients in the excellent range for all four reliability substudies. Taken together, the results of this study suggest that both the borderline diagnosis and the symptoms of BPD can be diagnosed reliably when using the DIB-R. They also suggest that excellent reliability, once achieved, can be maintained over time for both the syndromal and subsyndromal phenomenology of BPD.