INTRODUCTION: Mass-participation endurance events take place throughout the United Kingdom. Although out-of-hospital cardiac arrests (OHCAs) occur during these events, little is known about them. This case series aims to describe the number, type, etiology, and outcome of OHCAs treated by a UK-based specialist sports medicine provider over a period of 8 y. METHODS: The medical records of a UK-based sports medicine provider were reviewed from 2014 to 2022. Anonymized information from OHCAs during this time was recorded. This included type of event, patient demographics, details of OHCA, and patient outcomes. RESULTS: Ten OHCAs were identified during the course of 110 sporting events. These included the cases of 9 participants and 1 spectator. Return of spontaneous circulation (ROSC) was achieved on-site in all patients. Eight survived beyond 24 h and achieved a full neurological recovery. Seventy percent of these patients achieved ROSC within 4 min of cardiopulmonary resuscitation being initiated. The 2 patients who died both presented with a nonshockable rhythm. CONCLUSIONS: OHCAs during mass-participation endurance events are rare. However, medical providers must be prepared to respond promptly. Quick interventions can result in a full neurological recovery.