BACKGROUND: Whole blood trauma resuscitation is conceptually appealing and increasingly used but lacks evidence. A randomized controlled trial is needed but challenging to design. A Bayesian approach might be more efficient and more interpretable than a conventional frequentist design. We report the results on an elicitation meeting to create prior probability distributions to help develop such a trial. METHODS: In-person expert elicitation meeting, based on Sheffield Elicitation Framework methodology. We used an interactive graphical tool to elicit the quantities of interest (24-hour mortality and certainty required). Two rounds were conducted, with an intervening discussion of deidentified responses. Individual responses were aggregated into probability distributions. RESULTS: Fifteen experts participated. The pooled belief was that the median 24-hour mortality of trauma patients with hemorrhagic shock treated with component therapy (the current standard of care) was 19% (95% credible interval [CrI], 6%-45%), and the median 24-hour mortality of those treated with whole blood, 16% (95% CrI, 5%-39%). The pooled prior distribution for the relative risk had a median of 0.84 (95% CrI, 0.26-3.1), indicating that the expert group had a 64% prior belief that whole blood decreases 24-hour mortality compared to component therapy. CONCLUSIONS: Experts had moderately strong beliefs that whole blood reduces the 24-hour mortality of trauma patients with hemorrhagic shock. These data will assist with the design and planning of a Bayesian trial of whole blood resuscitation, which will help to answer a key question in contemporary transfusion practice.