BACKGROUND: Much research worldwide is focussed on cost containment and better adherence to guidelines in healthcare. The research focussing on professional behaviour is often performed in a well-controlled research setting. In this study a large-scale implementation of a peer review strategy was tested on both test ordering and prescribing behaviour in primary care in the normal quality improvement setting. METHODS: We planned a cluster-RCT in existing local quality improvement collaboratives (LQICs) in primary care. The study ran from January 2008 to January 2011. LQICs were randomly assigned to one of two trial arms, with each arm receiving the same intervention of audit and feedback combined with peer review. Both arms were offered five different clinical topics and acted as blind controls for the other arm. The differences in test ordering rates and prescribing rates between both arms were analysed in an intention-to-treat pre-post analysis and a per-protocol analysis. RESULTS: Twenty-one LQIC groups, including 197 GPs working in 88 practices, entered the trial. The intention-to-treat analysis did not show a difference in the changes in test ordering or prescribing performance between intervention and control groups. The per-protocol analysis showed positive results for half of the clinical topics. The increase in total tests ordered was 3% in the intervention arm and 15% in the control arm. For prescribing the increase in prescriptions was 20% in the intervention arm and 66% in the control group. It was observed that the groups with the highest baseline test ordering and prescription volumes showed the largest improvements. CONCLUSIONS: Our study shows that the results from earlier work could not be confirmed by our attempt to implement the strategy in the field. We did not see a decrease in the volumes of tests ordered or of the drugs prescribed but were able to show a lesser increase instead. Implementing the peer review with audit and feedback proved to be not feasible in primary care in the ...