Background: Outcome heterogeneity, selective reporting, and choosing outcomes that do not reflect needs and priorities of stakeholders, limit the examination of health intervention effects, particularly in late phase trials. Core outcome sets (COS) are a proposed solution to these issues. A COS is an agreed-upon, standardised set of outcomes that should be measured and reported as a minimum in all trials in a specific area of health or healthcare. COS are intended to increase standardisation of outcome measurement and reporting to better enable comparisons between, and synthesis of findings of trials in a particular health area. Methods: This study will examine late phase trials, published between October 2019 and March 2020 (inclusive), in the following five medical journals: New England Journal of Medicine, Journal of the American Medical Association, Lancet, BMJ, and Annals of Internal Medicine. Trials will be examined to determine if they refer to a COS, and whether they use a COS. Trialists for each identified trial will subsequently be contacted to complete an online survey examining trialists' awareness of, and decisions to search for and use a COS. Discussion: This study will provide important information on uptake of COS by later phase trialists in major medical journals, and the views of these trialists on COS use in trials. These findings will inform approaches to increasing awareness and uptake of COS in future health trials.