The ability of noni juice to improve endurance in athletes was evaluated clinically. A placebo-controlled clinical trial with TAHITIAN NONI® Juice (TNJ) was conducted in 40 highly-trained athletes. Drinking 100 mL of TNJ twice daily increased endurance (time-to-fatigue) by 21%, and improved antioxidant status as measured by a 25% decrease in blood chemiluminescence. Chemical analyses by multiple laboratories and drug-urine screening of human volunteers reveal that TNJ does not contain any illegal drugs or substances prohibited by the World Anti-doping Agency. The collective results indicate that TNJ improves endurance via potent antioxidant effects. The results warrant human clinical trials to assess the dosages and the feasibility of using TNJ by athletes participating in strenuous sports such as football, rugby, ice hockey, soccer and basketball.