Research question: The aim of the research was to determine and assess Polish long-distance running events’ (half marathons’) risk factors (the timespan of runs was limited to 2017 and 2018).?Research methods: Ten major half marathons held in 2017-2018 in Poland were considered. Ten experts carried out the multi-stage risk factor identification for the organization of a large-scale run with the use of the Delphi method, while 21 practitioners - representatives of the ten events’ organizers conducted risk assessment. Furthermore, the Cronbach’s alpha statistics was applied to establish the questionnaire’s reliability. The analysis was concluded with the factor analysis of the most significant risks.?Results and Findings: In the context of the outcome and probability of a given risk factor the following risks appeared to be dominant: financing marathons, organizational issues (including the start date), the acceptance of the event by local community, the organization of the finish line, delayed start, cyber security, disclosure of personal data and problems with the measurement of time.?Implications: The risk assessment results proved to be ideal input data for efficient sport events’ risk management, especially in the context of mass runs. Professional risk management based on the risk factors database created on the basis of the research most frequently leads to providing the highest possible safety and satisfaction levels for all stakeholders.