Background Despite increased ultramarathon participation in recent years, little is known about causes of race incompletion. This study surveyed runners of three 161–168 km ultramarathons in Europe and North America to explore characteristics and issues that affected race performance.

Objective Determine human and environmental factors related to failure to complete ultramarathons of ∼161 km in the previous year.

Design Observational study using surveys prior to the race.

Setting The 2013 Ultra-Trail du Mont-Blanc^® (UTMB^®) in Europe; Leadville Trail 100 (LT100) and Western States Endurance Run (WSER) in North America.

Participants Out of 4 110 race entrants, 2 794 (69%) completed the pre-race survey.

Risk factor assessment In this epidemiologic study, we report the incidence of intrinsic and extrinsic factors related to failure to finish of at least one ∼161 km race in the previous year.

Main outcome measurements The main outcome measurements were self-reported reasons for not finishing the race.

Results Among respondents, about 30% had masters or higher degrees. Out of 2,469 UTMB^®, 1206 LT 100, and 408 WSER entrants, the majority successfully completed the race (68%, 52% and 72%, respectively). Among participants who responded, 18.3% reported they failed to complete an ultramarathon of ∼161km in the past year. The main reasons for dropping out were inability to make the cut off time (23.1%), nausea and/or vomiting (16.5%), injury during the race (16.5%), and an ongoing injury (13.3%).

Conclusions We conclude that primary performance-limiting issues in 161–168 km ultramarathons include inability to make the cut off time, nausea and/or vomiting, and injury before and during the race.