Studies among adult patients have found that subthreshold borderline personality disorder (BPD) features are associated with elevated psychosocial morbidity compared with patients with no BPD features. However, the clinical significance of subthreshold features of BPD has not been investigated among real-world patients during the clinical emergence of the disorder, which is usually between puberty and emerging adulthood. This study aimed to replicate and extend previous research by comparing outpatient youth aged 15–25 years with subthreshold BPD features with youth with no BPD features. The sample included 499 potential participants, of whom 111 had no DSM-IV BPD features at all, and 155 had between one and four features. Results indicated that the group with subthreshold BPD features had more severe mental illness and poorer social and occupational functioning. These findings suggest that subthreshold BPD features are clinically important and should be a focus of clinical intervention to reduce continuing disability and improve outcome.