• age at onset;
  • bipolar disorder;
  • borderline personality disorder;
  • childhood abuse;
  • trauma

Objectives:  The relationship between bipolar disorder and cluster B personality disorders remains phenomenologically complex and controversial. We sought to examine the relationship between early age at onset of bipolar disorder and development of comorbid borderline personality disorder.

Methods:  A total of 100 adults in an academic specialty clinic for bipolar disorder underwent structured diagnostic interviews and clinical assessments to determine lifetime presence of comorbid borderline personality disorder, histories of childhood trauma, and clinical illness characteristics.

Results:  Logistic regression indicated that increasing age at onset of bipolar disorder was associated with a lower probability of developing comorbid borderline personality disorder (odds ratio = 0.91, 95% confidence interval: 0.83–0.99) while controlling for potential confounding factors, including a history of severe child trauma/abuse.

Conclusion:  Early onset of bipolar disorder increases the probability of developing comorbid borderline personality disorder, independent of the effects of severe childhood trauma/abuse. In patients with borderline personality disorder, prospective studies of new-onset bipolar disorder may underestimate the prevalence of true comorbidity unless they capture the primary risk window for first-episode mania arising before the end of adolescence.