Abstract Background The NOURISHED study: Nice OUtcomes for Referrals with Impulsivity, Self Harm and Eating Disorders. Eating Disorders (ED) and Borderline Personality Disorder (BPD) are both difficult to treat and the combination presents particular challenges. Both are associated with vulnerability to loss of mentalization (awareness of one’s own and others’ emotional state). In BPD, Mentalization Based therapy (MBT) has been found effective in reducing symptoms. In this trial we investigate the effectiveness and cost-effectiveness of MBT adapted for Eating disorders (Mentalization Based Therapy for Eating Disorders (MBT-ED)) compared to a standard comparison treatment, Specialist Supportive Clinical Management (SSCM-ED) in patients with a combination of an Eating Disorder and either a diagnosis of BPD or a history of self-harm and impulsivity in the previous 12 months. Methods/Design We will complete a multi-site single-blind randomized controlled trial (RCT) of MBT-ED vs SSCM-ED. Participants will be recruited from three Eating Disorder Services and two Borderline Personality Disorder Services in London. Participants allocated to MBT-ED will receive one year of weekly group and individual therapy and participants allocated to SSCM-ED will receive 20 sessions of individual therapy over 1 year. In addition, participants in both groups will have access to up to 5 hours of dietetic advice. The primary outcome measure is the global score on the Eating Disorders Examination. Secondary outcome measures include total score on the Zanarini BPD scale, the Object Relations Inventory, the Depression Anxiety Stress Scales, quality of life and cost-effectiveness. Measures are taken at recruitment and at 6 month intervals up to 18 months. Discussion This is the first Randomised Controlled Trial of MBT-ED in patients with eating disorders and symptoms of BPD and will provide evidence to inform therapy decisions in this group of patients. During MBT-ED mentalization is encouraged, while in SSCM-ED it is not overtly addressed. This study will help elucidate mechanisms of change in the two therapies and analysis of therapy and interview transcripts will provide qualitative information about the conduct of therapy and changes in mentalization and object relations. Trial registration ISRCTN51304415