Psychology and Psychotherapy: Theory, Research and Practice (2016), 89, 163–180 © 2015 The British Psychological Society www.wileyonlinelibrary.com Prospective cohort feasibility study of a transdiagnostic group intervention for common mental health problems: The Take Control Course Lydia Morris 1 *, Warren Mansell 1 , Richard Emsley 2 , Rachel Bates 3 , Jody Comiskey 3 , Emma Pistorius 3 and Phil McEvoy 3 1 School of Psychological Sciences, University of Manchester, UK 2 Centre for Biostatistics, University of Manchester, UK 3 Six Degrees Social Enterprise, CIC, The Angel Centre, Salford, UK Objectives. While transdiagnostic psychological treatments appear to be promising, they require greater empirical support. Further, a number of available transdiagnostic treatments are targeted at clients with a specific category of disorder, such as clients with anxiety disorders. This study is a preliminary examination of the effectiveness, feasibility, and acceptability of a new transdiagnostic six-session group-based intervention (Take Control Course; TCC) predominantly aimed at clients within primary care. The group is aimed at a broad range of clients; it is derived from an integrative transdiagnostic theory, which specifies mechanisms of psychopathology across disorders. Briefer interventions are gaining an increasing evidence base, and this study seeks to compare the TCC to an established brief intervention within primary care. Design. Prospective cohort study comparing two active psychological treatments. Methods. Take Control Course group (n = 66) was compared to a non-randomized control group of clients accessing individual low-intensity interventions (n = 43) using random-effect regression models. Primary outcomes were depression and anxiety scores; additional outcomes included social and other functioning. Results. For the TCC group, changes on all pre–post outcomes were significant with moderate effect sizes. The between-group differences were not significant. Conclusions. Results show potential for TCC to be an effective intervention, but further work is required to validate these findings in a more rigorous, randomized study. Practitioner points  Transdiagnostic understandings of psychological distress may inform pragmatic and effective treatments that can be offered to a broad range of clients.  This study describes a transdiagnostic intervention (TCC) that targets maintenance processes common across disorders, and presents initial outcome data.  The TCC was found to reduce pre–post scores on measures of anxiety and depression. *Correspondence should be addressed to Lydia Morris, School of Psychological Sciences, Coupland 1 Building, Oxford Rd, University of Manchester, Manchester M13 9PL, UK (email: lydia.morris-2@postgrad.manchester.ac.uk). DOI:10.1111/papt.12070 163