Saunders KEA, Jones T, Perry A, et al. - As per systematic reviews, nearly 20% of individuals who have bipolar disorder present with comorbid borderline personality disorder, however, current diagnostic systems support a move towards dimensional rather than categorical approaches to classifying personality pathology. In this study, the presence and severity of borderline personality traits were examined in bipolar I and bipolar II disorder, and relationships between the presence/severity of borderline personality traits and clinical outcomes were explored in bipolar disorder. Using the Borderline Evaluation of Severity over Time (BEST) questionnaire, 1,447 individuals with DSM-IV bipolar disorder (1,008 bipolar I disorder and 439 bipolar II disorder) were assessed for borderline personality traits. They observed the common occurrence of borderline personality traits in both bipolar disorder groups, with 86.2% participants describing at least one trait. Significantly more frequent and more severe borderline personality traits were evident in bipolar II disorder vs bipolar I disorder. In both bipolar disorder groups, more severe borderline traits, and even the presence of a single borderline personality trait, were significantly linked with younger age of bipolar disorder onset and higher prevalence of lifetime alcohol misuse. Based on outcomes, they recommend considering the presence of comorbid borderline personality traits in the management of all patients with bipolar disorder irrespective of whether criteria for a categorical borderline personality disorder diagnosis are met.