Symptomatic disorders often co-occur with borderline personality disorder (BPD). This study's purpose was to compare the rates of comorbidity reported by adult and adolescent inpatients with BPD, including complex comorbidity (i.e., a combination of disorders of affect and impulsivity). One hundred four adolescents (aged 13-17) and 290 adults (aged 18-35) with BPD were interviewed using an age-appropriate semistructured interview for the assessment of symptomatic disorders. Lifetime rates of mood disorders and ADHD were quite similar for the two study groups. However, rates of anxiety disorders, including PTSD, substance use disorders, eating disorders, and complex comorbidity were significantly higher among adults than adolescents. Taken together, the results of this study suggest that broadly defined disorders of both affect and impulsivity are more common among adults than adolescents with BPD. They also suggest that a pattern of complex comorbidity is even more distinguishing for these two groups of borderline patients.