BACKGROUND: There are significant barriers to accessing effective psychological therapy in primary care resulting from a lack of suitably trained therapists to meet current demand. More efficient service delivery using minimal interventions (such as bibliotherapy) provided by paraprofessional therapists may be one method of overcoming these problems, and is the subject of attention in the UK and elsewhere. A randomized trial was conducted to test the clinical effectiveness of this model. Assistant psychologists delivered a guided self-help intervention to patients with anxiety and depression who were currently waiting for psychological therapy. METHOD: A total of 114 patients were randomized either to guided self-help or a waiting-list control group. All patients were followed up 3 months later, prior to starting conventional psychological therapy. Measures included self-reported adherence to the intervention, anxiety and depressive symptoms, social functioning and patient satisfaction. RESULTS: Adherence to the guided self-help intervention was acceptable and patients reported satisfaction with the intervention. However, there were no statistically significant differences between groups in anxiety and depression symptoms at 3 months. CONCLUSIONS: The results demonstrate that this model of guided self-help did not provide additional benefit to patients on a waiting list for psychological therapy. The results are considered in the context of possible internal and external validity threats, and compared with previous trials of minimal interventions. The implications of the results for the design of future minimal interventions are considered.