Despite its legality, caffeine remains on WADA's monitoring program, which results in the drug be placed under heightened scrutiny for further tracking of trends in use and possible abuse for future prohibition list consideration1. This study assessed the knowledge and prevalence of caffeine use by athletes completing at the 2005 Ironman Triathlon World Championships. Caffeine-related questionnaires were administered to 140 (105 male and 35 female, 40.3ᱰ.7 years) athletes (8% of all participants) from 16 countries throughout race registration. 50 of these athletes further consented to immediate post-race blood samples for analysis of plasma caffeine and paraxanthine using HPLC. 100 (71%) athletes correctly identified caffeine as being unrestricted in triathlon. Only 15 (11%) athletes were not planning on using a caffeinated substance immediately prior to or throughout the race. Cola (78%), Caffeinated Gels (42%), Coffee (usually pre-race) (37%), Energy Drinks (13%), and No Doz Tablets (9%) were the most popular caffeinated choices. More than half the athletes (52%) stated that they did not know an optimal caffeine dose for their sport. The most common sources for information on caffeine were reported as self researched/experimentation (44%), fellow athletes (42%), magazines (38%) and journal articles (33%). Meanᓄ (and range) post race plasma caffeine and paraxanthine levels were 22.3Ჰ 孯l/L (1.7-98.4) and 9.4ᶠ孯l/L (1.8-28.9) respectively. Seven athletes (14%) finished with plasma caffeine levels =40 ?mol/L. Plasma values from elite athletes did not differ from age group competitiors. Caffeine is popularly targeted as a potential ergogenic aid by endurance triathletes. Surprisingly, despite the prevalence of its consumption and the training experience of this athletic group many participants remained either confused or misinformed of its legality. Levels of plasma caffeine and paraxanthine taken immediately post race indicate that athletes completing in ironman races often finish the race with quantities of caffeine that have been shown to improve endurance performance (i.e. 蠲0孯l/L or a dose of ? 3 mg/kg bw). Average paraxanthine levels would indicate that the initiation of caffeine ingestion was typically =3-4 hrs prior to race completion. The widespread use of caffeine by ironman athletes appears largely the result of self research and peer experimentation rather than by consultation with qualified professionals.