Background Illness-related medical encounters (illMEs) are common in mass community-based participation cycling events, but there are limited data on risk factors associated with illMEs in endurance cycling events.

Objective To determine the independent risk factors associated with illMEs in a mass community-based endurance cycling event.

Design Retrospective, cross-sectional study.

Setting 2012–2014 Cape Town Cycle Tour (109 km), South Africa.

Participants 102251 race starters.

Assessment of Risk Factors All illMEs and a subgroup of serious illMEs for 3 years were recorded and also grouped into common illnesses by organ system affected. The following possible risk factors associated with illMEs were explored: sex, age, cycling speed and environmental exposure [calculated as average individual Wet-Bulb Globe Temperature (aiWBGT)].

Main Outcome Measures Independent risk factors associated with illMEs, serious illMEs, and illMEs by organ system affected using a Poisson regression model.

Results Independent risk factors associated with all illME during an endurance cycling event were slow cycling speed (P=0.009) and higher aiWBGT (P< 0.001). Risk factors associated with serious and life-threatening or death ME were older age (P = 0.007) and slower cycling speed (P = 0.016). Risk factors associated with specific common illME were fluid and electrolyte disorders (females, older age, and higher aiWBGT) and cardiovascular illness (older age).

Conclusions Females, older age, slower cycling speed, and higher aiWBGT were associated with illME in endurance cycling. These data could be used to design and implement future prevention programs for illME in mass community-based endurance cycling events.