People with personality disorder experience long waiting times for access to psychological treatments, resulting from a limited availability of long-term psychotherapies and a paucity of evidence-based brief interventions. Mentalisation-based treatment (MBT) is an efficacious therapeutic modality for personality disorder, but little is known about its viability as a short-term treatment.

We aimed to evaluate mental health, client satisfaction and psychological functioning outcomes before and after a 10-week group MBT programme as part of a stepped-care out-patient personality disorder service.

We examined routinely collected pre–post treatment outcomes from 176 individuals (73% female) aged 20–63 years, attending a dedicated out-patient personality disorder service, who completed MBT treatment. Participants completed assessments examining mentalising capacity, client satisfaction, emotional reactivity, psychiatric symptom distress and social functioning.

Post-MBT outcomes suggested increased mentalising capacity (mean difference 5.1, 95% CI 3.4–6.8, P < 0.001) and increased client satisfaction with care (mean difference 4.3, 95% CI 3.3–5.2, P < 0.001). Post-MBT emotional reactivity (mean difference −6.3, 95% CI −8.4 to −4.3, P < 0.001), psychiatric symptom distress (mean difference −5.2, 95% CI −6.8 to −3.7, P < 0.001) and impaired social functioning (mean difference −0.7, 95% CI −1.2 to −0.3, P = 0.002) were significantly lower than pre-treatment. Improved mentalising capacity predicted improvements in emotional reactivity (β = −0.56, P < 0.001) and social functioning (β = −0.35, P < 0.001).

Short-term MBT as a low-intensity treatment for personality disorder was associated with positive pre–post treatment changes in social and psychological functioning. MBT as deployed in this out-patient service expands access to personality disorder treatment.

Two types of mentalisation-based treatment (MBT) have been developed and empirically evaluated for borderline personality disorder (BPD): day hospital MBT (MBT-DH) and intensive out-patient MBT (MBT-IOP). No trial has yet compared their efficacy.

To compare the efficacy of MBT-DH and MBT-IOP 18 months after start of treatment. MBT-DH was hypothesised to be superior to MBT-IOP because of its higher treatment intensity.

In a multicentre randomised controlled trial (Nederlands Trial Register: NTR2292) conducted at three sites in the Netherlands, patients with BPD were randomly assigned to MBT-DH (n = 70) or MBT-IOP (n = 44). The primary outcome was symptom severity (Brief Symptom Inventory). Secondary outcome measures included borderline symptomatology, personality functioning, interpersonal functioning, quality of life and self-harm. Patients were assessed every 6 months from baseline to 18 months after start of treatment. Data were analysed using multilevel modelling based on intention-to-treat principles.

Significant improvements were found on all outcome measures, with moderate to very large effect sizes for both groups. MBT-DH was not superior to MBT-IOP on the primary outcome measure, but MBT-DH showed a clear tendency towards superiority on secondary outcomes.

Although MBT-DH was not superior to MBT-IOP on the primary outcome measure despite its greater treatment intensity, MBT-DH showed a tendency to be more effective on secondary outcomes, particularly in terms of relational functioning. Patients receiving MBT-DH and MBT-IOP, thus, seem to follow different trajectories of change, which may have important implications for clinical decision-making. Longer-term follow-up and cost-effectiveness considerations may ultimately determine the optimal intensity of specialised treatments such as MBT for patients with BPD.