During the competitive season of 1990, 155 British triathletes whose competitive distances varied from sprint to full ironman, and who self-classified themselves as recreational, intermediate or élite, kept a training diary for an 8-week period. They gave details of injuries sustained while training for, or competing in, triathlons. The mean(s.d.) distances covered each week were: swimming, 4.2(2.6) km; cycling, 100.2(70.6) km; and running 23.4(15.2) km; mean(s.d.) training time was 7(3.4) h per week, and a mean(s.d.) of 7.9(3.4) training sessions were completed per week. At least one injury was reported by 37% of the participants. The most frequently affected sites were the ankle/foot, thigh, knee, lower leg and the back. Overuse was the reported cause in 41% of the injuries, two-thirds of which occurred during running. The likelihood of an injury was positively associated with experience in triathlon. Average injury rate was 5.4 injuries per 1000 h of training (95% confidence interval: 4-7.2) and 17.4 per 1000 h of competition (95% confidence interval: 10.9-27.9). Injury incidence was unrelated to the mean amount of weekly training or competition, intensity or frequency of training.