BACKGROUND: There is a lack of well-designed randomized controlled trials (RCTs) to investigate the efficacy of psychological therapies for children in foster care with emotional and behavioural difficulties. Mentalization-based therapy (MBT) focuses on supporting the carer-child relationship by promoting reflective capacity. This study examined the feasibility and acceptability of an RCT of MBT, delivered in a family-format, for children who are in foster care in the UK. METHOD: Herts and Minds was a phase II, blinded feasibility RCT with follow-up of at 12 and 24 weeks post-randomisation. Participants were children (age 5-16) in foster care referred to a targeted mental health service, who had some level of difficulty as identified by the Strengths and Difficulties Questionnaire (SDQ). Aims were to assess: the feasibility of recruitment processes and study uptake; capacity to train mental health practitioners to deliver MBT to an acceptable level of treatment integrity; establish acceptability and credibility of MBT as an intervention for children in foster care; establish feasibility and acceptability to participants of conducting an RCT; and estimate the likely treatment efficacy effect size. Participants were randomly allocated to either MBT (n = 15) or Usual Clinical Care (UCC) (n = 21) individually or in sibling groups. A range of qualitative and quantitative data was gathered to assess feasibility. RESULTS: Feasibility was established with regard to: capacity to recruit participants to a study; capacity to train mental health practitioners to deliver MBT to an acceptable level of treatment integrity; acceptability and credibility of MBT; and feasibility and acceptability to participants of conducting an RCT. A number of issues made it difficult to estimate a likely treatment efficacy effect size. CONCLUSION: With modifications, it is feasible to run an RCT of MBT for children in foster care. Both the therapy and research design were acceptable to participants, but modifications may be needed regarding both the timing of assessments and the identification of appropriate primary outcome measures. Given the lack of evidenced based therapies for this population, such a trial would be a significant contribution to the field. Findings may be useful for other groups planning clinical trials of psychological therapies for children in foster care. TRIAL REGISTRATION: ISRCTN 90349442 . The trial was retrospectively registered on 6 May 2016.