The ergogenic effect of caffeine on endurance exercise is commonly accepted. We aimed to elucidate realistically the effect of caffeine on triathlon event performance using a field study design, while allowing investigation into potential mechanisms at play. A double-blind, randomized, crossover, field trial was conducted. Twenty-six triathletes (14 males, 12 females) participated (age: 37.8±10.6 years, habitual caffeine intake: 413±505 mg/day, percentage body fat: 14.5±7.2%, training/week: 12.8±4.5 hours). Microencapsulated caffeine (6 mg/kg body weight) was supplemented 60 minutes pre-trial. Performance data included time to completion (TTC), rating of perceived exertion (RPE) and profile of mood states (POMS). Blood samples taken before, during and post-race were analyzed for cortisol, testosterone and full blood count. Capillary blood lactate concentrations were assessed pre-race, during transitions and 3, 6, 9, 12, 15 minutes after triathlons. Caffeine supplementation resulted in a 3.7% reduction in swim time (33.5±7.0 vs. 34.8±8.1 minutes, p<0.05) and a 1.3% reduction in TTC (149.6±19.8 vs. 151.5±18.6 minutes, p<0.05) for the whole group. Gender differences and individual responses are also presented. Caffeine did not alter RPE significantly, but better performance after caffeine supplementation suggests a central effect resulting in greater overall exercise intensity at the same RPE. Caffeine supplementation was associated with higher post-exercise cortisol levels (665±200 vs. 543±169 nmol/l, p<0.0001) and facilitated greater peak blood lactate accumulation (ANOVA main effect, p<0.05). We recommend that triathlon athletes with relatively low habitual caffeine intake may ingest 6 mg/kg body weight caffeine, 45-60 minutes before the start of Olympic-distance triathlon in order to improve performance.