The ability of beta-carotene (BC) to reduce lipid peroxidation in humans was investigated. In this randomized double-blind controlled trial, 42 nonsmokers and 28 smokers received either 20 mg BC or placebo daily for 4 wk. Twenty-five smokers and 38 nonsmokers completed the trial. Changes in plasma BC concentrations increased significantly (P < 0.0005) and to the same extent in both groups supplemented with BC. There were no significant changes among the placebo groups. At baseline, lipid peroxidation measured by breath-pentane output (BPO) was significantly higher in the two smoking groups (BC: 8.8 +/- 1.1, placebo: 9.4 +/- 1.4 pmol.kg-1.min-1) than in the two nonsmoking groups (BC: 5.7 +/- 0.5, placebo: 5.9 +/- 0.6 pmol.kg-1.min-1) (P < 0.005). BPO decreased significantly only in smokers receiving BC (6.5 +/- 0.7 pmol.kg-1.min-1) (P < 0.04). Changes in breath-ethane output were not significant. Therefore, lipid peroxidation measured by BPO is significantly higher in smokers than in nonsmokers and is reduced by BC supplementation in smokers. There was no significant change (95% CI - 1.26, 1.12) in BPO when nonsmokers received BC.