The primary aim of this study is to investigate clinical effectiveness of mindfulness training versus treatment as usual (TAU) in adults with Attention Deficit Hyperactivity Disorder (ADHD). The secondary aim is to assess whether mindfulness training is cost-effective compared to TAU in adults with ADHD from a societal perspective.

RATIONALE Attention Deficit Hyperactivity Disorder (ADHD) is a common neurodevelopmental disorder with a high persistence into adulthood. Patients with ADHD are primarily offered stimulant medication. However, not all patients are willing to take medication, some suffer from unacceptable side-effects and for many medication does not reduce their symptoms to the degree they would wish for. Therefore, there is a strong need for effective psychosocial interventions that are both accessible to a large group of patients and have been shown to be cost-effective, such as mindfulness training.

OBJECTIVE To examine the (cost)effectiveness of mindfulness versus treatment as usual (TAU) in adults with Attention Deficit Hyperactivity Disorder (ADHD).

HYPOTHESIS Mindfulness training will result in less ADHD symptoms, a better executive functioning, a better quality of life, and lower health care and societal costs.

STUDY DESIGN Randomised trial comparing mindfulness in addition to TAU with TAU alone. Baseline, end of treatment, follow-up 1 (6 months after baseline) and follow-up 2 (9 months after baseline) assessments will be done by blinded assessors and self-report questionnaires. After 9 months, patients allocated to the TAU condition will be offered mindfulness as well.

STUDY POPULATION N=120 adults with ADHD according to the DSM-5 using a structured psychiatric interview, referred to Radboudumc in Nijmegen, Reinier van Arkel in 's Hertogenbosch and Dimence in Deventer.

INTERVENTION The investigators have developed a treatment protocol of mindfulness for ADHD based on both the Mindfulness-Based Cognitive Therapy (MBCT) (Segal, Williams & Teasdale, 2013) and the mindfulness training for ADHD developed by Zylowska (2012).

STANDARD INTERVENTION TO BE COMPARED TO Treatment as usual, usually consisting of psychostimulants and psycho-education.

COST-EFFECTIVENESS ANALYSIS/ BUDGET IMPACT ANALYSIS Annual health care and societal cost savings are expected to be €1,2 million and €15,4 million, respectively.

Inclusion Criteria:

The investigators will include patients of 18 years and older who meet the following criteria:

• a primary diagnosis of ADHD, according to the criteria of Diagnostic and Statistical Manual of Mental Disorders - 4th edition (DSM-IV-TR) based on a structured Diagnostic Interview for ADHD, in adults (DIVA)
• capable of filling out questionnaires in Dutch

Exclusion Criteria:

• depressive disorder with psychotic symptoms or suicidality
• active manic episode
• borderline or antisocial personality disorder
• substance dependence
• autism spectrum disorder
• tic disorder with vocal tics
• learning difficulties or other cognitive impairments
• former participation in a MBCT or MBSR course