Background It is important to identify risk factors associated with medical complications during ultra-marathons so that prevention programmes can be developed.

Objective To determine risk factors for medical complications during ultra-marathons.

Design Prospective study.

Setting Two Oceans ultra-marathon (56 km) races.

Participants 26 354 race starters.

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 4 years. Complications were subdivided according to the system that was affected and by final diagnosis. A Poisson regression model was used to determine risk factors for any medical complication and for more common specific complications.

Results Risk factors for medical complications during 56 km road races were less running experience (≤1 medal vs 2–4 medals, p=0.0097), and both fastest (<6 vs 6–7 min/km, p=0.0051) and slowest (>7 vs 6–7 min/km, p<0.0001) running pace category. Year of observation was also associated with risk of complications (2009 vs 2008, p=0.0176; 2009 vs 2010, p=0.0007; 2010 vs 2011, p=0.0112). Risk factors for specific common medical complications were: postural hypotension (slowest pace), serious exercise-associated muscle cramping (older age, fastest pace), gastrointestinal complications (slowest pace) and dermatological complications (fastest pace).

Conclusions Less experience and running at either a slow or a fast pace were risk factors for complications during 56 km road running. Annual variation may also affect risk. Risk factors for specific medical complications were also identified. These data form the basis of further studies to assist medical staff to plan appropriate care at races.