OBJECTIVE Although the DSM-IV suggests that dysfunctional personality patterns can be traced back to adolescence, there is continued debate about whether borderline personality disorder (BPD) can be reliably and validly diagnosed before age 18. The current study examined the reliability and validity of BPD in a large sample of adolescent psychiatric patients. METHODS BPD and Axis I disorders were assessed with validated structured interviews and a series of clinical, emotion, and personality correlates were assessed with validated self-report questionnaires. RESULTS Consistent with previous studies in adolescent clinical samples, approximately 30% of patients in the current sample met criteria for BPD. The nine BPD criteria demonstrated good internal consistency, equivalent to rates reported in adult samples. In addition, BPD was related to greater clinical severity and impairment as indexed by strong associations with all major Axis I disorders, as well as with dimensional measures of depression, anxiety, difficulties with emotion regulation, and impulsiveness. Notably, reliability and validity remained satisfactory even when analyses were limited to younger adolescents between the ages of 12 and 14. CONCLUSIONS Overall, findings suggest that BPD can be reliably and validly diagnosed in adolescents as young as 12-14 years old.