Background The half-marathon (21 km) race is a very popular mass community-based distance running event. It is important to determine risk factors for medical complications during these events, so that prevention programmes can be developed.

Objective To determine risk factors associated with medical complications during 21 km road running events.

Design Prospective study.

Setting Two Oceans half-marathon (21 km) races.

Participants 39 511 starters in the 21 km race.

Methods Medical complications (defined as any runner requiring assessment by a doctor at the race medical facility or a local hospital on race day) were recorded over a 4-year study period. Medical complications were subdivided according to the system affected and by final diagnosis. A Poisson regression model was used to determine risk factors for any medical complication and more common specific complications.

Results Independent risk factors for medical complication during 21 km running were older female runners (women >50 vs  ≤50 years; p<0.0001) and year of observation (2008 vs 2011; p=0.0201: 2009 vs 2011: p=0.0019; 2010 vs 2011: p=0.0096). Independent risk factors for specific common medical complications were: postural hypotension (women, slow running pace), musculoskeletal complications (less running experience, slower running pace) and dermatological complications (women).

Conclusions Older female runners are at higher risk of developing medical complications during 21 km road running races. Environmental conditions in a particularly cold climate may also play a role. Less running experience and slower running pace are associated with specific medical complications. Medical staff can now plan appropriate care on race days, and interventions can be developed to reduce the risk of medical complications in 21 km races.