ABSTRACT: 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 mumol/mL in the 29.5 mL dose group (P < 0.01) and from 0.26 +/- 0.22 to 0.18 +/- 0.11 mumol/mL in the 118 mL dose group (P < 0.001). LOOH levels decreased from 0.53 +/- 0.19 to 0.40 +/- 0.10 mumol/mL in the 29.5 mL dose group (P < 0.001) and from 0.55 +/- 0.21 to 0.40 +/- 0.14 mumol/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.