OBJECTIVE: This study aims to calculate the global warming potential, in carbon dioxide (CO2) equivalent emissions, from all in-scope activities involved in a phase-1 clinical study. DESIGN: Retrospective analysis. DATA SOURCE: Internal data held by Janssen Pharmaceuticals. STUDIES INCLUDED: Janssen-sponsored TMC114FD1HTX1002 study conducted between 2019 and 2021. MAIN OUTCOME: Measure CO2 equivalents (CO2e) for in-scope clinical trial activities calculated according to intergovernmental panel on climate change 2021 impact assessment methodology. RESULTS: The CO2e emissions generated by the trial were 17.65 tonnes. This is equivalent to the emissions generated by driving an average petrol-fueled family car 71 004 km or roughly 1.8 times around the circumference of the Earth. Commuting to the clinical site by the study participants generated the most emissions (5419 kg, 31% of overall emissions), followed by trial site utilities (2725 kg, 16% of overall emissions) and site staff travel (2560 kg, 15% of overall emissions). In total, the movement of people (participant travel, site staff travel and trial site staff travel) accounted for 8914 kg or 51% of overall trial emissions. CONCLUSIONS: Decentralised trial models which seek to bring clinical trial operations closer to the participant offer opportunities to reduce participant travel. The electrification of sponsor vehicle fleets and society's transition towards electric vehicles may result in further reductions. TRIAL REGISTRATION NUMBER: NCT04208061.