Background Triathlon is an increasingly popular sport at both the elite and the recreational level. However, there have been few high-quality studies of injuries and illnesses among triathletes.Objective To register overuse problems and acute injuries among iron-distance triathletes throughout a training season leading up a major event.Methods A 26-week prospective cohort study was conducted including 174 participants of the 2011 Norseman Xtreme Triathlon. Data on overuse injuries located in the shoulder, lower back, thigh, knee and lower leg were collected every second week using the OSTRC Overuse Injury Questionnaire. Illnesses, acute injuries and overuse problems affecting other anatomical areas were also recorded using standard injury surveillance methods. Results The average prevalence of overuse problems was 56% (95% CI: 51-61) (490 cases). The average prevalence of substantial overuse problems was 20% (95 % CI: 18-21) (165 cases). The most prevalent sites of overuse problems were the knee (25%), lower leg (23%) and lower back (23%). The acute injury incidence was 0.97 injuries per 1000h of training (36 cases) and 1.02 injuries per 1000h of competition (5 cases). A majority of moderate and severe acute injuries were located at the knee, shoulder/clavicle and sternum/ribs. The predominant types of acute injuries were contusions, fractures and sprains.Conclusion Overuse problems constitute the majority of injury cases among iron-distance triathletes. The most prevalent sites of injury in the present study were the knee, lower leg, lower back and shoulder. Future injury prevention studies in iron-distance triathletes should focus on these areas.