BackgroundNo accepted intervention exists for borderline personality disorder presenting in adolescence.AimsTo compare the effectiveness of up to 24 sessions of cognitive analytic therapy (CAT) or manualised good clinical care (GCC) in addition to a comprehensive service model of care.MethodIn a randomised controlled trial, CAT and GCC were compared in out-patients aged 15–18 years who fulfilled two to nine of the DSM–IV criteria for borderline personality disorder. We predicted that, compared with the GCC group, the CAT group would show greater reductions in psychopathology and parasuicidal behaviour and greater improvement in global functioning over 24 months.ResultsEighty-six patients were randomised and 78 (CAT n=41; GCC n=37) provided follow-up data. There was no significant difference between the outcomes of the treatment groups at 24 months on the pre-chosen measures but there was some evidence that patients allocated to CAT improved more rapidly. No adverse effect was shown with either treatment.ConclusionsBoth CAT and GCC are effective in reducing externalising psychopathology in teenagers with sub-syndromal or full-syndrome bipolar personality disorder. Larger studies are required to determine the specific value of CAT in this population.