Aims: Much research has begun to focus on the identification of people who are at high risk of developing psychosis, and clinical services have been initiated for this population. However, only a small number of studies have reported on the efficacy of interventions for preventing or delaying the onset of psychosis. The results of prior work suggest that cognitive therapy (CT) may be an effective, well-tolerated treatment. We report on the rationale and design for a large-scale, multi-site randomized, controlled trial of CT for people who are assessed to be at high risk of psychosis because of either state or state-plus-trait risk factors. Methods: The study employs a single-blind design in which all participants receive frequent mental-state monitoring, which will efficiently detect transition to psychosis, and half are randomized to weekly sessions of CT for up to 6months. Participants will be followed-up for a minimum of 12months and to a maximum of 2years. Results: We report the characteristics of the final sample at baseline (n=288). Conclusions: Our study aimed to expand the currently limited evidence base for best practice in interventions for individuals at high risk of psychosis. © 2011 Blackwell Publishing Asia Pty Ltd.