Background. Noni (Morinda citrifolia) juice has demonstrated antioxidant activity in vitro and in vivo. To evaluate this activity in humans, noni juice from Tahiti (TNJ) was evaluated in a 30 day, double-blind, and placebo controlled clinical trial with 285 current heavy smokers. Research participants were randomly assigned to three daily treatment groups: 118 mL placebo, 29.5 mL TNJ, and 118 mL TNJ. Plasma superoxide anion radicals (SAR) and lipid hydroperoxide (LOOH) levels were measured pre and post-intervention. Results. After 30 days, mean SAR decreased from 0.26±0.14 to 0.19±0.10 mol/mL in the 29.5 mL dose group (P < 0.01) and from 0.26±0.22 to 0.18± 0.11 mol/mL in the 118 mL dose group (P < 0.001). LOOH levels decreased from 0.53±0.19 to 0.40±0.10 mol/mL in the 29.5 mL dose group (P < 0.001) and from 0.55±0.21 to 0.40±0.14 mol/mL in the 118 mL dose group (P < 0.001). No significant reductions in SAR or LOOH levels were observed in the placebo group. Conclusion. The results suggest an antioxidant activity from noni juice in humans exposed to tobacco smoke, thereby replicating the results found previous chemical and in vivo tests.