OBJECTIVE: To calculate the global warming potential, in carbon dioxide (CO(2)) equivalent emissions, from a sample of pragmatic randomised controlled trials. DESIGN: Retrospective analysis. Data source Internal data held by NIHR Evaluation, Trials and Studies Coordinating Centre. Studies included All eligible pragmatic randomised controlled trials funded by the NIHR Health Technology Assessment programme during 2002 and 2003. MAIN OUTCOME MEASURE: CO(2) equivalents for trial activities calculated with standard conversion factors. RESULTS: 12 pragmatic randomised controlled trials involving more than 4800 participants and a wide range of technologies were included. The average CO(2) emission generated by the trials was 78.4 (range 42.1-112.7) tonnes. This is equivalent to that produced in one year by approximately nine people in the United Kingdom. Commuting to work by the trial team generated the most emissions (average 21 (11.5-35.0) tonnes per trial), followed by study centres' fuel use (18 (9.3-32.2) tonnes per trial), trial team related travel (15 (2.0-29.0) tonnes per trial), and participant related travel (13 (0-46.7) tonnes per trial). CONCLUSIONS: CO(2) emissions from pragmatic randomised controlled trials are generated in areas where steps could be taken to reduce them. A large proportion of the CO(2) emissions come from travel related to various aspects of a trial. The results of this research are likely to underestimate the total CO(2) emissions associated with the trials studied, because of the sources of information available. Further research is needed to explore the additional CO(2) emissions generated by clinical trials, over and above those generated by routine care. The results from this project will feed into NIHR guidelines that will advise researchers on how to reduce CO(2) emissions.