BACKGROUND: Data Monitoring Committees (DMCs) are increasingly involved in the conduct of randomized controlled trials, but there is little documented evidence of what they do. Three interlinked surveys were carried out as part of the DAMOCLES project to explore recent and current DMC practice and policy. METHODS: 1) A questionnaire about DMC practice was sent to sample of 45 authors of trials published in selected journals in 2000. The sample was stratified by centre (single/multiple), disease area, and presence of DMC. 2) A sample of investigators in trials ongoing in the United Kingdom in 2001-02 was also sent a questionnaire about DMC practice. The sample was drawn from trials funded by the Medical Research Council, the United Kingdom Department of Health's Health Technology Assessment Programme, and a local and a multicentre research ethics committee. The sample was additionally stratified by funder (public/industry), centre (single/ multiple), and disease area. 3) A sample of major organisations involved in randomised controlled trials was sent a questionnaire about DMC policies. RESULTS: Information about DMC practice from the first survey was obtained from 31 trials (69%), of which four had a DMC. Information about DMC practice from the second survey was obtained about 36 trials (90%), of which 20 had a DMC. Information about DMC policy from the third survey was obtained from 25 out of 25 organisations. There was general agreement about the sorts of trials particularly needing independent DMCs, but there were few uniform approaches to their modes of functioning, and few of the organisations surveyed had developed formal policies. CONCLUSIONS: The roles of existing DMCs and policies governing DMC functioning vary widely across trials and organisations that sponsor or oversee trials, both within the UK and internationally. These findings reinforce previous calls for development of such policies across a wider range of organisations, better means to monitor their implementation within trials, and wider use of structured "charters", which set out DMC modus operandi in advance. Clinical Trials 2005; 2: 22-33. www.SCTjournal.com