BackgroundLittle is currently known about the presence and impact of personality disorder in adolescents who self-harm.AimsTo evaluate personality disorder in repeated self-harm in adolescence and its impact on self-harm psychopathology and adaptation outcomes over 1 year.MethodA clinical referral sample (n = 366) of adolescents presenting with repeated self-harm aged 12-17 years, as part of a randomised controlled trial (Assessment of Treatment in Suicidal Teenagers study, ASSIST). Personality disorder was assessed using the Structured Clinical Interview for DSM-IV Axis II (SCID-II). One-year outcomes included frequency and severity of repeat self-harm, self-reported suicidality, mood and functional impairment.ResultsAbout 60% of the referred adolescents showed one or more forms of personality disorder. Personality disorder was associated with significantly greater severity of self-harm, overall psychopathology and impairment. There was a complex association with treatment adherence. Personality disorder predicted worse 1-year outcomes in relation to self-harm frequency and severity, as well as impairment, suicidality and depressive symptoms.ConclusionsPersonality disorder can be reliably measured in adolescence and showed high prevalence in this clinical self-harm sample. Controlling for other variables, it showed a strong independent association with self-harm severity at referral and predicted adherence to treatment and clinical outcomes (independent of treatment) over 1 year. Consideration of personality disorder diagnosis is indicated in the assessment and management of adolescents who repeatedly self-harm.