Contains Video 1 Acceptance and Commitment Therapy for a Heterogeneous Group of Treatment-Resistant Clients: A Treatment Development Study Sue Clarke, Bournemouth University Jessica Kingston, Royal Holloway, University of London Kelly G. Wilson, University of Mississippi Helen Bolderston and Bob Remington, University of Southampton Acceptance and commitment therapy (ACT) has been shown to have broad applicability to different diagnostic groups, and there are theoretical reasons to consider its use with clients with chronic mental health problems. We report an innovative treatment development evaluation of ACT for a heterogeneous group of ”treatment-resistant clients” (N = 10) who had attended a mean of 3.5 previous psychological interventions. All clients had Axis I presentations and half met diagnostic criteria for Axis II disorders. Functioning, assessed at pre- and postintervention, and at 6- and 12-month follow-up, showed improvements over time on all primary outcome measures, driven largely by significant changes occurring between baseline and 6-month follow-up. Improvements were associated with ACT processes of change. The data thus suggest that a broad range of clients who had not benefited from standard care may benefit from ACT. A range of psychotherapeutic methods have been used with marked success to treat acute psychological disorders. Unfortunately, however, evidence from clinical trials shows that a proportion of clients fail to obtain clinically meaningful improvements following such in- terventions, continue to experience persistent symptoms, and thus remain resistant to treatment. For example, approximately 30 to 60% of clients fail to obtain improve- ments following cognitive behavior therapy (CBT) for generalized anxiety disorder (Borkovec, Newman, Pincus, & Lytle, 2002), depression (DeRubeis et al., 2005; Dimidjian et al., 2006), bulimia nervosa (Wilson, Fairburn, & Agras, 1997), and mixed symptoms (Westbrook & Kirk, 2005). Similarly inconsistent results have been reported following interpersonal therapy for depression (Elkin et al., 1989) and bulimia nervosa (Agras, Walsh, Fairburn, Wilson, & Kraemer, 2000), and following psychodynamic therapy for anorexia nervosa (Dare, Eisler, Russell, Treasure, & Dodge, 2001) and depression (Leichsenring, 2001). From among this sizeable minority of treatment-resistant clients, many will present in clinics with a broad array of symptomatology, often including chronic, comorbid, and personality-disor- dered symptoms (Fournier et al., 2008). Thus, there is an urgent need for cost-effective, evidence-based psychological interventions for treatment- resistant clients experiencing a heterogeneous range of clinical disorders. The primary purpose of the present study was to develop a group-based treatment for this transdiag- nostic group. In contrast to approaches that focus on specific diagnoses, this intervention was developed to target the psychological processes underlining a range of treatment-resistant presentations. As such, it represented an important initial step toward the development of a pandiagnostic intervention for challenging patients. In the last 15 years, new therapeutic interventions have been developed that, unlike traditional CBT, are less concerned with modifying the content or frequency of clients’ private events (e.g., distressing thoughts and emotions), and instead focus on teaching them to accept these events without treating them as literally true. Dialectical behavior therapy (DBT; Linehan, 1993a), acceptance and commitment therapy (ACT; S. C. Hayes, Strosahl, & Wilson, 1999), and mindfulness-based cogni- tive therapy (MBCT; Segal, Williams, & Teasdale, 2002) Keywords: acceptance and commitment therapy; group-based inter- vention; heterogeneous client group; treatment development; treatment-resistant clients 1077-7229/11/xxx-xxx$1.00/0 © 2012 Association for Behavioral and Cognitive Therapies. Published by Elsevier Ltd. All rights reserved. 1 Video patients/clients are portrayed by actors. CBPRA-00419; No of Pages 13: 4C Please cite this article as: Clarke et al., Acceptance and Commitment Therapy for a Heterogeneous Group of Treatment-Resistant Clients: A Treatment Development Study, Cognitive and Behavioral Practice (2012), doi:10.1016/j.cbpra.2012.03.001 Available online at www.sciencedirect.com Cognitive and Behavioral Practice xx (2012) xxx-xxx www.elsevier.com/locate/cabp